The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster. The Influence of Intermolecular Interaction on Intramolecular Vibrations

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1 Z. Phys. Chem. 218 (2004) by Oldenbourg Wissenschaftsverlag, München The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster. The Influence of Intermolecular Interaction on Intramolecular Vibrations By A. S. Gemechu 1,L.J.H.Hoffmann 1, S. Marquardt 1, C. G. Eisenhardt 1, H. Baumgärtel 1,, R. Chelli 2, G. Cardini 2, and S. Califano 2 1 Institute of Chemistry, Physical and Theoretical Chemistry, Free University Berlin, Takustr. 3, Berlin, Germany 2 Dipartimento di Chimica, University of Florence, via G. Capponi 9, Florence, Italy Dedicated to Prof. Dr. Herbert Zimmermann on the occasion of his 75 th birthday (Received October 25, 2003; accepted October 25, 2003) Anisole / Anisole/CO 2 / Quantumchemical Calculations / REMPI Spectra / Geometry / Vibrations The S 1 ( 1 B 2 ) S 0 ( 1 A 1 ) electronic transition of anisole and the anisole/co 2 aggregate cooled in a supersonic free jet has been investigated in detail using REMPI spectroscopy and quantumchemical model calculations. The 42 intramolecular modes of anisole in the 1 S 1 state are assigned. Some previous assignments of modes of anisole in the 1 S 1 state have been improved, some of the assignments are still tentative. The origin of the corresponding electronic transition in the 1:1-aggregate is blueshifted by 117 cm 1 versus the 0-0 transition of anisole, the origine in the 1:2-agreggate is redshifted by 216 cm 1. Probably a second conformer of the 1:1-aggregate is formed in the molecular beam. 36 fundamental modes of the anisole/co 2 1:1-cluster out of possible 46 intramolecular modes are assigned. Spectral shifts of the fundamental modes in the 1 S 1 state of anisole/co 2 due to the aggregation have been observed. The intermolecular modes and their binary combinations with intramolecular modes have been analyzed. 1. Introduction The intermolecular interaction has been of interest since the early days of physical chemistry. One of the most popular examples is the equation of state of real gases. * Corresponding author. baum@chemie.fu-berlin.de

2 124 A. S. Gemechu et al. Intermolecular interaction results from the simultaneous action of different intermolecular forces. The nature of these forces are well-known and have been described in details [1]. The intermolecular forces modify genuin properties of molecules. In the frame of this work we will report on the influence of weak intermolecular interaction on the vibrational structure of the participating molecules anisole and carbondioxide in the anisole/co 2 1:1-complex. The combination of molecular beam technique with laser spectrocopy allows the detailed experimental study in which way aggregation of these molecules influences their intramolecular vibrations in the 1 S 1 state. In addition quantumchemical model calculations of the systems reveal the influence of intermolecular interaction on the intramolecular modes in the electronic ground state. Molecular aggregates of aromatic systems with small molecules are very suitable subjects for this kind of work. Therefore, numerous different molecular clusters have been investigated by laser spectroscopy. The results are documented in several informative reviews [2 7]. Considering small 1:1- and 1:2-aggregates, many of the investigations focused on systems with hydrogen bonds. Typical examples are clusters of phenol with water, alcohols, amines etc. [8 16]. Many of the experiments were focused on the determination of intermolecular vibrational modes which image the intermolecular interaction. The interpretation of the spectroscopic results was supported by quantumchemical calculations. They give informations on the total energy, the geometry and vibrations in the 1 S 0 state. However, so far the influence of aggregation on the intramolecular vibrations of the aromatic chromophore has not been investigated systematically, although the influence of solvents in solutions or solid matrices on the energy of vibrational modes has been observed since the early days of IR spectroscopy. In these early experiments small solvent shifts could not be assigned to a molecular aggregate with specific structure. Mass selected REMPI spectra offer the possibility to study vibrational modes in the 1 S 1 state and their shifts in size selected aggregates. In the electronic ground state the assignment of fundamental modes can be supported by quantumchemical model calculations, whereas the assignment of these modes in the 1 S 1 state is much more difficult. Firstly, the accuracy of routine quantumchemical calculations for this state is still insufficient and, secondly, it is well known that fundamental modes may be considerably shifted in the excited state in comparison to the ground state. Therefore the analysis of vibrational modes in the excited electronic state remains difficult and the tentative character of assignments has to be taken into account. In this paper we will present a comparison between the REMPI spectra of anisole and anisole/co 2. Spectroscopically anisole is a large system, but a first attempt will be made to assign the 42 intramolecular vibrations of anisole in the 1 S 1 state, because this is of key importance for forthcoming experiments with clusters containing anisole.

3 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Fig. 1. Scheme of the experimental setup. The paper is organized in the following way: First we give a short report on the experimental setup, followed by the results of model calculations on anisole/co 2 in the electronic ground state. Then the REMPI spectra of anisole and anisole/co 2 (1:1) will be analyzed and discussed. For briefness we compile the results in tables. The figures illustrate parts of the spectra which are of interest for the discussion. 2. Experimental setup A schematic drawing of the experimental setup is given in Fig. 1. It has been described in detail previously [17, 18]. It consists of a supersonic beam coupled to a time of flight spectrometer, a tunable dye laser and a data aquisition system. The time of flight tube can be used for the analysis of ions as well as of electrons. Details of the construction have been given earlier [18, 19]. The stagnation pressure for the expansion of the gas mixtures (He/anisole and He/anisole/CO 2 ) has been varied between 0.2 and 2.5bar. We used a 50 mm nozzle and a 300 mm skimmer to admit the aggregates from the molecular beam source to the ionization chamber. The clusters are ionized by the frequency-doubled output of a Nd-YAG pumped dye laser (Lambda Physics Scanmate 2EC-400 OG) calibrated with a neon OG lamp. Coumarin 153 and Coumarin 307 dissolved in methanol have been used as dyes. The resolution in the REMPI spectra generally was 0.1cm 1, some energy ranges (not shown here) have been measured with cm 1 using an etalon.

4 126 A. S. Gemechu et al. 3. Ab initio calculations The calculations were carried out by using the density functional theory (DFT) [20] as inplemented in the GAUSSIAN 98 package [21]. In DFT good results are achieved by using smaller basis sets than required in other correlated methods. However, it is important to choose an appropriate combination of basis set and exchange-correlation functionals as shown by Rauhut and Pulay [22] and by Scott and Radom [23]. The combination of the 6-31G(d) basis set [24] with B3-LYP exchange-correlation functional represents a good compromise between accuracy and computer time cost. Of course, the agreement of the calculations with experimental data can be improved by using a larger basis set. Therefore we used the B3-LYP functional and the 6-31G++(d,p) basis set. The very tight convergence criteria have been adopted for the minima localization. A few starting configurations have been choosen, they all converged either in the global energy minimum A or at higher energy in a second minimum B. A very fine grid has been used for all the calculations to increase the accuracy of the second derivative. The B3-LYP functional is defined in terms of the Dirac Slater (DS), Hartree Fock (HF), Becke (B88) [25], Lee Yang Parr (LYP) [26] and Vosko Wilk Nussair/VWN) [27] functionals according to the expression: F B3-LYP = 0.8F x (DS) + 0.2F x (HF) F x (B88) F c (LYP) F c (VWN). A satisfactory fit of the experimental frequencies in the electronic ground state is obtained by scaling the calculated frequencies in the range up to 2000 cm 1 by the factor and those in the higher frequency range by the factor Results and discussion 4.1 Geometry and symmetry of anisole and anisole/co 2 The geometry of anisole and of its clusters with small molecules is of considerable interest when this system is considered as a model for the study of intermolecular interactions. There are several experimental investigations [28] and ab initio calculations [29, 30] which reveal that the equilibrium conformation of anisole in the electronic ground state is planar. This configuration is energetically favored because of the conjugation of one of the lone pair electrons of the methoxy group with the aromatic π-system. However steric effects may constrain the system in a nonplanar conformation. The analysis of the very high resolution spectrum of the 1 S 1 1 S 0 transition of anisole [28] confirmes the planar geometry of anisole. In the 1 S 1 state, however, there are changes of the geometry in comparison to the electronic ground state. The C O CH 3 angle increases by 2 with

5 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Fig. 2. Structure of the anisole/co 2 -aggregates. Isomer A (on top) is more stable than isomer B (below). respect to the neutral molecule. In addition, the length of the two C C bonds adjacent to the C O bond increases, whereas the two next C C bonds decrease and the other two C C-bonds increase. These changes indicate the tendency to a more quinoidal structure. Quantumchemical calculations show that this tendency continues in the anisole ion [29]. The geometry of the anisole/co 2 1:1-cluster has been obtained from quantumchemical model calculations. In many cases the potential energy surface of molecular aggregates shows several different local minima. The geometry of the minimum energy anisole/co 2 1:1-aggregate (conformer A) in the 1 S 0 state as obtained from our calculations is shown in Fig. 2. The results of the model calculations and the results from a high resolution spectroscopy study of the 0-0 transition of this complex [31] fit together very well. The complex A is planar and the geometry of anisole and CO 2 are not changed in comparison to the isolated compounds. In the electronic ground state the center of

6 128 A. S. Gemechu et al. mass distance of anisole and carbondioxide comes to 44.3 pm and the angle α between the pseudo-c 2 axis (C(6) C(3) O(12)) of anisole and the axis of carbondioxide is about 40.Inthe 1 S 1 state the center of mass distance between the two molecules comes to 44.7pm and α is increased by 15.Therotation of the methoxy group in the aggregate generates two rotational isomers. The rotational barrier in isolated anisole is 3.9 kj/mol [30]. In the aggregate free rotation is not possible because of steric hindrance by the carbondioxide molecule. The model calculations revealed a second minimum corresponding to an anisole/co 2 -aggregate B with a slightly altered structure (Fig. 2). The total energy of aggregate B is 5.5kJ/mol higher than that of the aggregate A. The population of the different minima in the molecular beam is a priori not known, but the conformer assigned to the global minimum may dominate among the different species with a given cluster size. The inspection of the REMPI spectrum of the 1:1-aggregate exhibits signals of low intensity which point to a small contribution of the structure B besides the more stable conformer A. From the structure of anisole follows that its molecular symmetry should in principle be classified according to Longuet Higgins theory as belonging to the G 4 group, since in anisole the O CH 3 group can undergo a large scale rotation about the C O bond. The G 4 group is isomorpheous with the C 2v group and has the same character table. According to previous papers we classify the skeleton modes to A 1, A 2, B 1, B 2 of the C 2v group, this makes the comparison with skeleton modes of other benzene derivatives easier. Fundamental modes of the methoxy group are classified to the symmetry species a and a of the C s group. For the vibrations of the aromatic system we apply the well-known Wilson notation [32]. The anisole/co 2 1:1-cluster belongs to the C s symmetry group, however, the chromophor in the complex is anisole and therefore from the spectroscopical point of view we prefer for the discussion the use of the C 2v symmetry of anisole. Finally it should be noticed that a very rigid application of symmetry selection rules for the analysis of the optical transitions in the cluster is not possible. 4.2 Vibrational structure of anisole and anisole/co 2 in the 1 S 0 state The vibrational structure of the electronic ground state of anisole is well characterized. The 42 normal vibrations have been assigned by IR- and Raman studies and by quantumchemical model calculations [29, 33]. Before entering the detailed discussion of the REMPI spectra the vibrational structure of the aggregates A and B (Fig. 2) according to the results of the model calculations of the 1 S 0 state shall be described. In the 1:1-aggregates one expects in addition to the 42 intramolecular modes of anisole five intermolecular modes and 4 intramolecular modes of CO 2. The values resulting from the model calculations are compiled in Table 1.

7 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Table 1. Calculated modes of intermolecular and intramolecular vibrations of anisole, anisole/co 2 (A) and anisole/co 2 (B). Nr. assignment anisole anisole/co 2 (A) anisole/co 2 (B) COC torsion, a b, B b, B O CH 3, torsion a a, A COC bending, a b, B a, A b, B ν 2 (CO 2 ) ν 2 (CO 2 ) , B , B C OCH 3 stretching, A a, A b, B a, A , B , A , A a, a b, B CH 3 rocking, a , B a, A CH 3 rocking, a a, A , B ν 1 (CO 2 ) , B CH 3 sym. deformation, a b, B CH 3 asym. deformation, a CH 3 asym. deformation, a a, A

8 130 A. S. Gemechu et al. Table 1. continued. Nr. assignment anisole anisole/co 2 (A) anisole/co 2 (B) 41 8b, B a, A ν 1 (CO 2 ) CH 3 sym. stretching, a CH 3 asym. stretching, a CH 3 asym. stretching, a b, A , B , A b, B a, A As expected, the intermolecular modes appear at very low energy. The intramolecular modes of anisole in the cluster are shifted in comparison to the isolated system. Due to the weak interaction between anisole and carbondioxide these spectral shifts are small. The maximum shifts are observed with mode 7a, which is downshifted in cluster A by 20 cm 1 and with mode 8b, which is upshifted in cluster A by 31 cm 1 in comparison to anisole. The differences between the frequencies of the intramolecular modes of anisole in the aggregates A and B are negligibly small. Another remarkable result of the calculations concernes the vibrations of CO 2 in the anisole/co 2 aggregates. The calculated values of the CO 2 modes are smaller in the anisole/co 2 aggregate than the experimental values of isolated CO 2. In the cluster A the bending mode ν 2 is not degenerate. The difference between the in plane and out of plane bending mode comes to 11 cm 1. However, in the cluster B the ν 2 mode of CO 2 remains degenerate. We have performed no calculations for the 1 S 1 state of anisole and the aggregates with CO 2 because we expect the accuracy of the results obtained with standard ab initio programs not to be sufficient for comparing the fundamental modes in anisole and the anisole/co 2 aggregates. Therefore the assignment of the REMPI signals to normal modes, as given below, is based on the comparison with data of related benzene derivatives. 4.3 The REMPI spectrum of anisole Anisole is spectroscopically a large system and consequently the REMPI spectrum exhibits many structural features even at medium spectroscopic resolution as used in this experiment.

9 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Fig. 3a. REMPI spectrum of anisole. The figures a e show parts of the spectrum in which most of the intramolecular modes appear. The number of aromatic molecules for which excited state vibrations have been assigned remains small in comparison to the large number of aromatic molecules for which ground state vibrations have been studied in detail. Consequently many of the excited state vibrational modes up to now remain unassigned. The REMPI spectrum of anisole due to overtones and combinations shows much more features than the signals of the 42 fundamental modes. It can be compared with the conventional gasphase spectrum [34]. This comparison reveals corresponding signals for many fundamental modes in the 1 S 1 state. However, the conventional spectrum measured at room temperature shows many features close to the origine which are not observed in the REMPI spectrum. The REMPI spectrum of anisole is the key spectrum for further investigations of clusters containing anisole and small molecules. Therefore strong efforts are made to determine the frequencies of fundamental modes in the 1 S 1 state. We measured the REMPI spectrum of anisole in the range cm 1. The lowest π π * transition ( 1 S 1 ( 1 B 2 ) 1 S 0 ( 1 A 1 )) is electric dipol allowed for single photon absorption. We observed the origin at ± 2cm 1. The spectrum is shown in Fig. 3a e and may be compared with the conventional absorption spectrum measured at room temperature by

10 132 A. S. Gemechu et al. Fig. 3b. continued. Balfour [34]. He finds the origin of the π π * transition at cm 1,the difference of these values may be due to the calibration in both experiments. The REMPI spectrum shows more signals than the spectrum reported by Balfour. In both spectra the main intensities are observed at 757 cm 1 and in the range cm 1 above the origin. The symmetry forbidden transitions to vibronic states with B 2 modes, the overtones and combinations generate a considerable number of weak signals. There are some differences between the REMPI spectrum and Balfours spectrum considering the wavenumbers of corresponding signals and some assignments. This will be adressed below in detail. Moreover there are differences between these two spectra due to the different temperatures of the probes. Even at the lowest expansion pressure the temperature in the molecular beam exceeds hardly 50 K. Therefore in the

11 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Fig. 3c. continued. Fig. 3d. continued.

12 134 A. S. Gemechu et al. Fig. 3e. continued. REMPI spectrum we observe only a few hot bands with very low intensity close to the origine. Balfour has assigned many signals to hot bands. In addition the spectral resolution in the REMPI spectrum is improved in comparison to Balfours spectrum. For the assignment of vibrational modes in the 1 S 1 state relevant data from the literature are helpful. The evaluation of vibrations in the 1 S 1 state by quantumchemical calculations has been reported in the literature [35 38]. The results of these calculations are valuable and may be used as a first approximation for vibrational mode shifting in going from the 1 S 0 to the 1 S 1 state. However, they are not precise enough to give reliable support for the assignment of the signals in the REMPI spectrum. To make the assignment of fundamental modes in the 1 S 1 state of anisole as confident as possible we compare our results with studies on p-dimethoxy-benzene [38], p- and m-difluorobenzene [39, 40] and other substituted benzenes. Generally most of the ring modes have lower frequencies in the 1 S 1 state than in the electronic ground state due to the occupation of an antibonding π * orbital which leads to ring expansion and lower force constants. Consequently one expects downshifts of the vibrational modes which are connected with motions of atoms involved in the π system. This tendency can be documented by shifts of the normal modes of benzene. In this molecule the modes 1, 19, 6, 9, 8

13 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster are downshifted in 1 S 1 in comparison to 1 S 0 by cm 1, mode 3 is only shifted by 23 cm 1, mode 15 is not altered (+2cm 1 ). A remarkable exception is the frequency upshift of the Kekule-typ mode 14 (b 2u ) in the 1 B 2u electronically excited state of several aromatic hydrocarbons and some of their derivatives. In benzene mode 14 is upshifted by 261 cm 1 in the 1 B 2u state versus the 1 A 1g electronic state. This unusual behaviour has been discussed by Y. Haas et al. [41]. They assume that the σ frame is essentially responsible for the D 6h structure of benzene, whereas the π electrons prefer a D 3h distortion. The transfer of an electron in the antibonding π * orbital increases the σ character of the benzene ring mode. This causes an increase in the force constant along this coordinate resulting in an increase of the corresponding frequency. Similar results have been reported for p-andm-difluorobenzene [40]. For stretching and bending vibrations which involve the COC substructure a small upshift may be expected because the C OCH 3 bond becomes stronger in the 1 S 1 state due to the increased interaction between the aromatic ring and the methoxy group. The C H stretching modes of the aromatic ring and the CH 3 group are hardly influenced by electronic excitation. However, a slight increase of the corresponding modes due to the increased force constant can not be excluded. The frequencies of the C H bending and torsional modes of the OCH 3 group are unchanged or slightly decreasing as anisol is excited to the 1 S 1 state. For the readers convenience details of the spectrum are discussed in four sections. The assignments of fundamental modes of anisole in the 1 S 1 state as proposed from the results of our measurement are compiled in Table Origin 900 cm 1 range In the electronic ground state one observes in this range the COC torsion mode, the COC bending mode, the O CH 3 torsion mode, the C OCH 3 stretching mode and the modes 10b, 9b, 16b, 16a, 6a, 4, 6b, 11, 17b, 10a, 17a. In the excited state one expects a comparable number of fundamental modes. According to the IR spectrum of anisole in the 1 S 0 state the COC torsion mode and mode 10b are expected to have very low energies. Balfour assignes to the COC torsion mode in the 1 S 1 statethevalue82.9cm 1. However in our measurements no signal has been observed which could be assigned unambigeously to the COC torsion mode. There are several weak signals around 200 cm 1. The weak signals at 234 cm 1 and 269 cm 1 are assigned tentatively to mode 10b and 16b. The signal observed at 258 cm 1 is in agreement with the value given by Balfour, who assigned it to the 18b mode. However, according to the results of our calculations it is more likely to assign this signal to the 9b mode. The weak signal at 269 cm 1 may be due to mode 16b, but this assignment is very tentative, in fact, there is another weak signal at 289 cm 1 which also

14 136 A. S. Gemechu et al. Table 2. Shifts of the fundamental modes in the 1 S 0 and the 1 S 1 state caused by the formation of the anisole/co 2 (1:1)-aggregate (A).* Sym. assignment anisole anisole/co 2 anisole Intens. anisole/co S0 S0 S1 S1 (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) rel.% (cm 1 ) (cm 1 ) calc. calc. exp. exp. A 1 6a, X sensitive C OCH 3 stretching (ring breathing) a a / a a b 3063 ± a B 2 9b X sensitive b b b b b * the error limit of experimental values is ±2cm 1

15 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Table 2. continued. Sym. assignment anisole anisole/co 2 anisole Intens. anisole/co S0 S0 S1 S1 (cm 1 ) (cm 1 ) (cm 1 ) (cm 1 ) rel.% (cm 1 ) (cm 1 ) calc. calc. exp. exp. A 2 16a a a B 1 10b b , X sensitive 669 ± b a COC bending O CH 3 stretch. (18a) CH 3 rocking CH 3 sym. deformation CH 3 asym. deformat CH 3 sym. stretching CH 3 asym. stretching a COC torsion ??? O CH 3 torsion CH 3 rocking CH 3 asym. deformat CH 3 asym. stretching

16 138 A. S. Gemechu et al. could be correlated with this mode. Balfour assignes to mode 16b a signal at 447 cm 1, however, we find no corresponding signal. The signal at 299 cm 1 is assigned to the O CH 3 torsional mode, which is found in the ground state at 266 cm 1. The shift to higher values may be explained by the increased C O bond order in the excited state. Balfour assigned to this energy the first overtone of mode 16a, but the intensity of the signal seems to be too high for an overtone. A weak signal at 384 cm 1 could be assigned to the excitation of mode 16a. The corresponding transition is symmetry forbidden in C 2v but the limitations by symmetry are not strictly valid. At 425 cm 1 we observe a signal which fits the value calculated for the COC bending mode (a )inthes 0 state (433 cm 1 ) reasonably well. This signal has not been reported by Balfour [34]. The modes 6a and 6b are clearly recognized at 500 cm 1 (553 cm 1 in 1 S 0 ) and at 526 cm 1 (618 cm 1 in 1 S 0 ). This assignment has already been given earlier. In benzene mode 6 is shifted from 608 cm 1 ( 1 S 0 ) to 521 cm 1 ( 1 S 1 ). Between these two signals two weaker signals at 507 cm 1 and 515 cm 1 are observed in the REMPI spectrum of anisole. The signal at 507 cm 1 is assigned to mode 4 which in the 1 S 0 state is observed at 669 cm 1. The weak signal at 515 cm 1 may be the first overtone of the 9b mode. The two signals at 651 cm 1 and 683 cm 1 are assigned to mode 11 and mode 17b, which are found in the 1 S 0 state at 752 cm 1 and 880 cm 1 respectively. At 706 cm 1 a weak signal is recognized which probably indicates the mode 10a, but this assignment is a very tentative one. In the 1 S 0 state this mode is represented by a weak signal at 819 cm 1. Another small signal appears at 749 cm 1 as a shoulder in front of the very strong signal at 757 cm 1. The small signal may be assigned to mode 17a, a C H wagging mode (956 cm 1 in 1 S 0 ). One of the strongest signals of the spectrum is observed at 757 cm 1.Balfour also finds this very strong signal at 759 cm 1 and assigns it to mode 12 whereas we prefer to this signal the designation C OCH 3 stretching mode. This difference is based on the results of our model calculations which reveal that both modes contribute significantly to this signal. In this paper mode 12 is assigned to a strong signal at 940 cm 1. Besides the signals discussed above one observes numerous signals with lower intensity which are caused by combinations and overtones of vibrations in the 1 S 1 state. We have used the experimental values of the fundamental modes in the excited state as given in Table 2 to calculate expected values of overtones and binary combinations. In Table 4 these expected values are compared with the signals observed in the REMPI spectrum. The calculated and the experimental values agree remarkably well within the error limit. This supports the values of fundamental modes given in Table 2, however, it supports not specific assignments.

17 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster cm cm 1 range In this narrow range a very characteristic accumulation of strong signals at 935 cm 1, 940 cm 1, 946 cm 1, 951 cm 1, 955 cm 1 is observed. Their assignment is still under discussion because relevant data are hardly available from the literature. Balfour reports in this range the fundamental modes 1 and 18a at 937.4cm 1 and 952.6cm 1, respectively. Smalley et al. [42] measured the REMPI spectrum of toluene and assigned signals at 933.4cm 1 to mode 12 and at 964.9cm 1 to 18a. In our previous work [29] we assigned two of the signals to the modes 12 and 1. Only few data on the frequency shifts in going from the 1 S 0 to the 1 S 1 state are available. In benzene the frequencies of modes 1, 12, 18a are lower in the 1 S 1 state than in 1 S 0 state (1: 70 cm 1, 12: 42 cm 1, 18a: 118 cm 1 ). Tzeng et al. [38] postulate small downshifts of the modes 18a ( 35 cm 1 ), 1 ( 35 cm 1 ), 12 ( 15 cm 1 )inthe 1 S 1 state of p-dimethoxybenzene in comparison to the 1 S 0 state. Taking into account similar shifts in anisole the contribution of modes 1, 12, 18a in this range is very reasonable. However, the number of strong signals still exceeds the number of modes discussed so far. There are two possible explanations for the two remaining signals. Firstly, additional signals in this range may be due to other modes. Secondly, the number of signals may be increased due to Fermi resonances. The wagging vibration 5 (975 cm 1 in 1 S 0 ) may be hardly influenced by the change from 1 S 0 to 1 S 1, therefore this mode may correspond to the signal at 955 cm 1. Another candidate is the O CH 3 stretching mode (1039 cm 1 in S 0 ) which could cause one of the strong signals in this range. Fermi resonance occurs only between a fundamental mode and a combination or between two combinations when the resonating transitions have the same symmetry. This is fullfilled for the modes 1, 12, 18a and 6a, which belong to A 1. However the first overtone of mode 6a is expected around 1000 cm 1 which seems to be too high for Fermi resonance with the other modes. On the other side Fermi resonance between the combination (17b + 16b) and mode 5 cannot be excluded. In summary, we favour the contribution of additional fundamental modes to this very characteristic accumulation of signals, however, we can not exclude that Fermi resonance plays also a role. A definite signal to mode assignment in this narrow range of the spectrum seems to be impossible at present cm cm 1 range In this range of the spectrum several signals can be assigned to fundamental modes of anisole. The signal at 994 cm 1 clearly correlates with a signal at 992.6cm 1 reported by Balfour [34], it can be assigned unambigeously to the mode 9a. There is a weak signal at 1127 cm 1 which seems to be too strong for an overtone or combination, but could represent mode 3.

18 140 A. S. Gemechu et al. The signal at 1140 cm 1 correlates well with the CH 3 rocking mode (a ), whichisobservedinthe 1 S 0 state at 1143 cm 1. The corresponding a mode is observed at 1179 cm 1 (1180 cm 1 in 1 S 0 ). Between these two signals at 1098 cm 1 a weak signal is recognized which, according to its shape, could be due to mode 15. At 1270 cm 1 and 1273 cm 1 two intensive signals appear which are followed by an intensive signal at 1287 cm 1. For the interpretation of these signals modes of the CH 3 group can be excluded. We assign the signals at 1270 cm 1 and 1273 cm 1 to mode 7a (1282 cm 1 in 1 S 0 ) which is down shifted and splitted by Fermi resonance with the combination of 6a and the C OCH 3 stretching mode. In p-dimethoxybenzene a small upshift has been reported [38] for this mode. The source of the signal at 1287 cm 1 may be the mode 19b, which is observed in the ground state at 1455 cm 1.Adownshiftof52cm 1 has been reported for this mode in p-dimethoxybenzene [38]. Between 1287 cm 1 and 1400 cm 1 a great number of weak overtones and combinations is observed (Table 3) cm cm 1 range In this part of the spectrum a considerable number of fundamental modes is expected and, in fact, one observes another accumulation of signals around 1450 cm 1, but with much lower intensity in comparison to the accumulation around 950 cm 1. Among the fundamental modes in this range there are the deformation modes of the CH 3 group. In the 1 S 0 state the following modes have been observed [33]: sym. deformation mode (a ) 1442 cm 1, asym. deformation mode (a ) 1452 cm 1 and asym. deformation mode (a ) 1469 cm 1. The frequency of these modes in the 1 S 1 state should be nearly the same. In the REMPI spectrum appear weak signals at 1443 cm 1,1455 cm 1, 1468 cm 1 which may be assigned to these deformation modes (Table 2). It should be mentioned that in this energy range the number of signals increases considerably, so that the rather weak signals of the CH 3 modes are embedded in an accumulation of signals with weak to medium intensity. Therefore, the error limit in the assignment of these modes may be somewhat higher than for the other fundamental modes. The remaining signals can be assigned unambigeously. The signal at 1517 cm 1 is assigned to the 19a mode (1497 cm 1 in 1 S 0 ). It is one of the strongest signals in the REMPI spectrum. The reason for the high intensity of this transition may be a Fermi resonance with the first overtone of the C OCH 3 stretching mode at 757 cm 1. For mode 19 in benzene one has observed a downshift of 79 cm 1 [43]. At 1528 cm 1 and at 1543 cm 1 we observe two signals, which can be assigned unambigeously to modes 8b and 8a. They are shifted to lower energies in comparison to the 1 S 0 state by 60 cm 1 and 56 cm 1, respectively.

19 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Table 3. Overtones and binary combinations of anisole and anisole/co 2 (A) in the 1 S 1 state*. anisole 1 S 1 anisole/ 1 S 1 CO 2 expected observed expected observed assignment A a a + C OCH 3 stretch a a a + 9a 1773/ a + 7a a + 19a a + 8a ? 6a + 7b a a + 20a (C OCH 3 ) C OCH C OCH 3 stretch C OCH 3 stretch. + 9a 2030/ C OCH 3 stretch. + 7a C OCH 3 stretch. + 19a C OCH 3 stretch. + 8a ? C OCH 3 stretch. + 7b a 2213/ a a a a 2208/ a a a ? a / / a + 7a 2511??? a + 19a a + 8a 2546/ a / a + 19a 2816/ /09 7a + 8a a a + 8a a 2 B b b + 6b b + 18b

20 142 A. S. Gemechu et al. Table 3. continued. anisole 1 S 1 anisole/ 1 S 1 CO 2 expected observed expected observed assignment ? b b ? b + 19b b + 8b b b b + 20b b b + 18b b b b + 19b b + 8b b b b + 20b b ? b b b + 19b b + 8b b ? ? b b b b ? b ? 19b + 8b ? 19b b b ? 14 2 A a a + 10a a + 17a a a + 17a a 2

21 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Table 3. continued. anisole 1 S 1 anisole/ 1 S 1 CO 2 expected observed expected observed assignment B b b + 16b b b b + 17b b b b sh b b + 17b b b ? b b b a (COC bend.) COC bend. + O CH 3 stretch COC bend. + CH 3 rock COC bend. + CH 3 sym. def COC bend. + CH 3 asym. def COC bend. + CH 3 sym. stretch COC bend. + CH 3 asym. stretch (O CH 3 stretch.) O CH 3 stretch. + CH 3 rock O CH 3 stretch. + CH 3 sym. def O CH 3 stretch. + CH 3 asym. def (CH 3 rock.) ??? 2545 CH 3 rock. + CH 3 sym. def CH 3 rock. + CH 3 asym. def (CH 3 sym. def.) CH 3 sym. def. + CH 3 asym. def (CH 3 asym. def.) 2 a ? (COC tors.) ? COC tors. + OCH 3 tors. 1274??? COC tors. + CH 3 rock COC tors. + CH 3 asym. def.

22 144 A. S. Gemechu et al. Table 3. continued. anisole 1 S 1 anisole/ 1 S 1 CO 2 expected observed expected observed assignment ? COC tors. + CH 3 asym. stretch (OCH 3 tors.) OCH 3 tors. + CH 3 rock OCH 3 tors. + CH 3 asym. def OCH 3 tors. + CH 3 asym. stretch (CH 3 rock.) CH 3 rock. + CH 3 asym. def (CH 3 asym. def.) 2 * The maximal error limit is ±4cm 1 ;? The difference between expected and experimental value exceeds the error limit;??? No corresponding signal in the spectrum; No measurement in this range of energy This correlates with the shift of mode 8 in benzene which is shifted down by 85 cm 1 in the 1 S 1 state [37, 45] in comparison to the 1 S 0 state. Fuson et al. [44] have assigned the more intensive signal at higher frequency in the spectrum of toluene to vibration 8a. A strong downshift of mode 8b by more than 500 cm 1 as proposed for p-dimethoxybenzene [38] can not be confirmed by our experiments. The prominent signal at 1571 cm 1 obviously is caused by the Kekule mode 14, for which strong upshifts have been observed in benzene and other aromatic molecules [41]. Balfour [34] has observed a signal at 1567 cm 1 which he assigned to mode 8a The 2900 cm cm 1 range In this range one expects the aromatic and aliphatic C H-stretching modes. We observe the sym. stretching mode (a )ofthech 3 group at 2965 cm 1 and the asym. stretching mode (a ) at 2976 cm 1. The positions of these modes are upshifted by a few wavenumbers in comparison to the values obtained for the 1 S 0 state, whereas the asym. stretching mode (a ) at 2982 cm 1 is downshifted by 23 cm 1. The aromatic C H-stretching modes show similar behaviour. Their wavenumbers in the 1 S 1 state are slightly enhanced in comparison to the 1 S 0 state. In the REMPI spectrum they are clearly recognized at 3064 cm 1 (7b), 3074 cm 1 (13), 3081 cm 1 (2), 3097 cm 1 (20b) and 3106 cm 1 (20a). Wavenumbers of numerous signals of overtones and combinations in this range of the spectrum are compiled in Table 4. They confirm the assignment of spectral features at lower wavenumbers to fundamental modes.

23 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster REMPI spectrum of anisole/co 2 Many REMPI spectra of complexes of aromatic molecules with rare gas atoms or with small molecules have been published [2 8]. However, generally the energy range which has been considered was rather small because the main interest was focused on the appearence of the intermolecular modes near the origin. We have measured the REMPI spectrum of anisole/co 2 (1:1) up to 1520 cm 1 above the origin because we are interested on the influence of the intermolecular interaction on the intramolecular modes of anisole in the 1 S 1 state and on the coupling of these modes with the intermolecular modes. For an approximative assessment of the mode shifting in the 1 S 0 state caused by weak intermolecular interaction we used the results of the quantumchemical calculations for the 1 S 0 state of anisole [29] and the anisole/co 2 1:1-cluster (Tables 1 and 2). For the cluster the calculation reveals the intermolecular and intramolecular vibrational modes. The mode shifting in the 1 S 1 state due to intermolecular interaction may be evaluated by comparing the REMPI spectra of pure anisole and of anisole/co 2, provided the assignment of the intramolecular modes in the 1 S 1 state of the cluster is possible. The REMPI spectrum of the anisole/co 2 1:1- aggregate exhibits more signals than the spectrum of anisole due to the coupling of intermolecular modes with intramolecular modes. The intensity of the signals obviously is influenced by dissociation of the ionized aggregate into the anisole cation and CO 2. Therefore we registered simultaneously the REMPI spectrum in the mass channel m/e = 92. This REMPI spectrum is not identical with the REMPI spectrum of pure anisole because both the ions of pure anisole and the fragment ions of the 1:1-cluster contribute to the spectrum. There is no disturbing contribution of the anisole/co 2 1:2-cluster. The REMPI spectrum of the anisole/co 2 aggregate will be analyzed in two steps: The range close to the origin (Fig. 4) where only intermolecular modes (Fig. 5) are expected and the range where intramolecular modes appear. Two characteristic parts of the REMPI spectrum are shown in Fig. 6a and 6b in comparison to the spectrum of pure anisole The intermolecular modes of anisole/co 2 The origin of the 1 S 1 1 S 0 transition of the anisole/co 2 complex A is blueshifted by 117 cm 1 in comparison to anisole and observed at cm 1. As already mentioned small amounts of the anisole/(co 2 ) 2 complex have been observed. The 0-0 transition of this complex appears at cm 1 i.e. redshifted by 216 cm 1 versus the corresponding transition of anisole. Due to the low intensity of the signals of anisole/(co 2 ) 2 and their redshift there is no significant interference between the REMPI spectra of these two complexes. Close to the origin the spectrum is dominated by the signals of intermolecular modes and their first overtones and combinations (Fig. 4). One recognizes

24 146 A. S. Gemechu et al. Fig. 4. REMPI spectrum of anisole/co 2 (1:1) close to the origin. The signal at cm 1 is assigned to the origin of the isomer B. the signals of the five intermolecular modes, their first overtones and combinations. The assignment of these signals is compiled in Table 4. A remarkable signal appears at cm 1. The position and intensity of this signal fit not into the frame of the intermolecular modes. Therefore we assume that this signal may be due to the origin of the 1 S 1 1 S 0 band of conformer B of the 1:1-aggregate. The observation that many of the strong signals have satellites blueshifted by 72 cm 1 supports this assumption. The results of the model calculations have shown that the frequencies of the intramolecular modes of anisole in both conformers are nearly identical, however, the intermolecular modes of conformer A and B are different (Table 1). The contributions of conformer B to the spectrum are not discussed due to the low intensity of the corresponding signals. Another signal which fits not in the pattern of intermolecular modes appears at 98 cm 1. We assigned it to the COC torsion mode (a )ofanisole. It is observed in the 1 S 0 state of anisole at 81 cm 1, however, it can not be localized unambigeously for the 1 S 1 state in the REMPI spectrum of pure anisole. In beams generated with low stagnation pressure (0.2 bar) one observes several hot bands. They reveal the wavenumbers of intermolecular modes in the 1 S 0 state, which can be compared directly with calculated values. The ex-

25 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Fig. 5. Intermolecular modes of isomer A as obtained from the model calculations.

26 148 A. S. Gemechu et al. Fig. 6. a,b: Comparison of the REMPI spectra of anisole/co 2 and anisole in the range cm 1 (6a) and cm 1 (6b) above the origine. The number of transitions in the spectrum of the aggregate exceeds by far that in the anisole spectrum.

27 The Absorption Spectrum of Anisole and the Anisole/CO 2 1:1-Cluster Table 4. Intermolecular modes of the anisole/co 2 aggregate A in the 1 S 1 state. expect. (cm 1 )** obs. (cm 1 ) assignment 25* ν 1 39* ν 2 45* ν 3 57* ν 4 80* ν ν ν 1 + ν ν 1 + ν ν 1 + ν ν 1 + ν ν ν 2 + ν ν 2 + ν ν 2 + ν ν ν 3 + ν ν 3 + ν ν ν 4 + ν ν 2 5 * Error limit: ±2cm 1 ; ** For comparison the values of the overtones and combinations calculated from the basic modes are compiled in this table. perimental values obtained in this way come very close to the values calculated for conformer A and the 1 S 0 state (Table 1). This confirms the assumption that the intermolecular vibrations in the 1 S 0 and the 1 S 1 state have similar frequencies Intramolecular modes The fundamental intramolecular modes in the REMPI spectrum of the aggregate are recognized due to their relatively strong intensity. In comparison to anisole the positions of the intramolecuar modes in the aggregate are shifted. As a first approximation the corresponding shifts in the electronic ground state as obtained from the model calculations (Table 1) can be used for the assignment. In addition the signals of overtones and combinations of fundamental modes are detected in the spectrum, their values confirm the assignment of signals to fundamental modes. In Table 3 overtones and the combinations belonging to the same symmetry are compiled. Another characteristic feature is the coupling of intramolecular modes with the five intermolecular modes which leads to five characteristic addi-

28 150 A. S. Gemechu et al. tional weak signals blueshifted from the signal assigned to the fundamental mode. The spectral shifts due to the interaction of anisole with carbondioxide are small. According to the quantumchemical model calculations in the 1 S 0 state the limits of the shifts are +31 cm 1 and 20 cm 1.Inthe 1 S 1 state these limits are +20 cm 1 and 23 cm 1. The amount of mode shifting is comparable in both states because the structure of the complex in both states is similar. These shifts are much smaller than the mode shifting in going from the electronic ground state to the excited state. There remain weak signals in the spectrum which are not assigned here. They may be due to symmetry forbidden transitions and the coupling of overtones and combinations with intermolecular modes. The vibrational structure of carbondioxide is also affected by the intermolecular interaction with anisole. This is clearly seen from the results of the model calculations. In isolated carbondioxide one observes three vibrational signals [46]: the doubly degenerate bending mode (ν 2 ) at cm 1, the symmetric (ν 1 ) and asymmetric stretching mode (ν 3 ) at cm 1 and cm 1 respectively. The model calculations reveal a considerable decrease of these values in the aggregate. The most interesting result of the calculations is the lifting of the degeneration of the bending mode in the conformer A. This mode splits in two components, the bending mode of CO 2 in the plane of the complex at 619 cm 1 and the bending mode perpendicular to the plane of the complex at 630 cm 1. In the conformer B the bending mode of CO 2 remains degenerate (626.7cm 1 and 626.6cm 1 ). For both conformers A and B the calculated values of the CO 2 stretching modes in the complex are 1313 cm 1 (ν 1 ) and 2322 cm 1 (ν 3 ). The vibrations of CO 2 can not be excited directly in our experiment, but if vibrational states of anisole in the excited electronic state are in resonance with the CO 2 modes indirect vibrational excitation of CO 2 is possible. Modes 6b (523 cm 1 ), 11 (643 cm 1 ) and 17b (675 cm 1 ) in conformer A are the modes next to the nondegenerate bendig modes of CO 2. This is an off resonance situation. However there exists the possibility that combinations of the bending modes of CO 2 with the intermolecular modes are in resonance with these modes. In fact within the error limits mode 11 of anisole fits to the combination of the CO 2 in plane bending mode with the intermolecular mode ν 1 and mode 17b fits to the combination of the in plane bending mode of CO 2 with the intermolecular mode ν 4 as well as to the combination of the out of plane CO 2 bending mode with the intermolecular mode ν 3. The combination of 6b with the COC torsional mode fits only to the 618.9cm 1 bending mode of CO 2.In the spectrum of the anisole/co 2 complex one observes two signals at 623 cm 1 and 630 cm 1. In isolated CO 2 the symmetric stretching mode ν 1 is influenced by Fermi resonance. In anisole/co 2 this mode is expected at 1313 cm 1 in the electronic ground state and Fermi resonance with the overtone of the bending mode can