Effect of bending vibration on rotation and centrifugal distortion parameters of XY2 molecules. Application to the water molecule

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1 LETTRES Ce It J. Physique 45 (1984) L11 L15 ler JANVIER 1984, L11 Classification Physics Abstracts 33.20E Effect of bending vibration on rotation and centrifugal distortion parameters of XY2 molecules. Application to the water molecule V. I. Starikov, B. I. Makhanchejev and V1. G. Tyuterev The Institute of Atmospheric Optics, USSR Academy of Sciences, Siberian Branch, Tomsk, , USSR (Re~u le 27 septembre 1983, accepte le 16 novembre 1983) 2014 Résumé. travail présente le calcul du comportement anormal des paramètres de rotation et de distorsion centrifuge de la molécule H2O à partir du potentiel Lorentz + quadratique d ordre zéro. Des cas particuliers des formules générales de dépendance de ces paramètres en fonction du nombre quantique v2 associé à la vibration de déformation angulaire sont discutés Abstract. The calculation of the anomalous behaviour of rotation and centrifugal distortion parameters of H2O molecule with a Lorentz + quadratic zeroorder potential is presented. Particular cases of general giving the dependence of these parameters with respect to the quantum number v2 associated with the bending vibration are discussed. 1. Introduction. The anomalous behaviour of rotation and centrifugal distortion parameters as a function of the bending quantum number V2 is one of the most interesting features in highresolution vibrationrotation spectroscopy of XY2 molecules. For the water molecule it was discussed first by CamyPeyret and Flaud [14] on the basis of an analysis of fitted data. An explanation of this behaviour was suggested in references 5, 6. Here we present the result of calculations of rotational and quartic centrifugal distortion using a Lorentz + quadratic potential function for the H20 molecule. v2dependent parameters in ef f ective rotational Hamiltonian. is convenient to choose intramolecular coordinates according to Hougen, Bunker and Johns (HBJ) [8] : p is the bending variable, Jp its associate momentum, ql and q3 are the normal coordinates associated with the small vibrations vi and V3. The Contact Transformations (CT) of the HBJHamiltonian to an effective rotational Hamiltonian which do not use any poorly convergent expansions in p are quite similar to those described in reference 7 for the ammonia molecule (see also [6]). Unlike the effective Hamiltonian used by Article published online by EDP Sciences and available at

2 L12 JOURNAL DE PHYSIQUE LETTRES Hoy and Bunker [9] (reviewed by Sorensen [10]), the operator (1) is a purely rotational one. It can be reduced to Watson s form [ 11 ] since ~ and r~ are constant but not functions of p as in [810]. On the contrary to the usual formulae for a semirigid molecule [12, 13] the leading contributions to the constants B and r depend on the bending quantum number t ~ v~ where the secondorder contributions have the form Secondorder formula for the centrifugal distortion constants reads [5, 6] : Here st, 8,, and t/j p t/j s are energies and wavefunctions of the bending states (0 t o), (0 s 0) The kinetic energy operator Tbend associated with the nv2 modes is that of reference Calculated parameters of the H20 molecule. In order to calculate the matrix elements appearing in equations 24 onehas to know all the functions of p~~ involved. According to Hougen et al. [8] the functions Ba( p) and 2~(p) are the following where the ~(p~~are given in reference 8. The inertia derivatives and Coriolis parameters take the forms

3 Calculated with the following notation v2dependence OF CENTRIFUGAL DISTORTION L13 The anharmonicity parameters Ki(p), Kijl(p) and wi(p) are calculated according to the scheme of reference 9 where the only leading term in the expansion A~ = L L~(p) Qk + has been k kept with the L~(p) functions given by The bending potential Vo(p) in equation 5 has been taken to be a Lorentz + quadratic potential function : Parameters in equation 9 are chosen in such a way that Et calculated from equation 5 restitutes the experimental vibrational energies. In figure la the dependence of effbi on the bending quantum number t V2 is presented for three values of barrier height. The higher the barrier (i.e. the = more rigid the molecule), the smoother the behaviour of B. A similar regularity takes place for other parameters as well. Fig. 1. behaviour of B,, and A as a function of the bending quantum number V2 for the water molecule. The lines correspond to direct calculations using Eqs. 24 (the calculated behaviour of B~, A K, A JK, d,, bj presented in the figure and in the table was not fitted to the experimental one). A Lorentz + quadratic potential function Vo(p) is used in the calculations with the following values of barrier height H, cm1, HI, cm1, HII, cm1 used = = = to reproduce the vibrational levels (0 t 0) (see references 8, 9, 5). Empty dots 0 and squares D correspond to experimental values of references 14.

4 Calculated One L14 JOURNAL DE PHYSIQUE LETTRES According to the wellknown Nielsen s ordering scheme (based on expansions of the inertia tensor and of the potential function in all vibrational coordinates [12,13]), the vibrational dependence of a spectroscopic parameter has to be two orders of magnitude less than the leading contribution. However the fitted parameters of H20 [14] associated with the operators j2" do not obey this scheme, they increase much more rapidly. Within the present approach the calculated V2 behaviour of the spectroscopic parameters [5, 6] is in agreement with the experimental one [14]. In table I we present the parameters having the strongest V2 dependence calculated with a Lorentz + quadratic potential. These are direct calculations performed without any adjustable parameter. Table I. (*) and experimental values of the parameters e"bz and A K for H2160. (*) The potential (9) with the parameters K = cm1, H = cm1, Ye = 7r Pe = is used; r 0 = A (see references 9, 5). 3. Particular cases of v2dependent formulae for spectroscopic parameters. In order to relate the present approach to more standard ones it is convenient to discuss the following particular cases of equation HARMONIC OSCILLATOR APPROXIMATION FOR THE BENDING VIBRATION IN THE ZEROORDER HAMILTONIAN (4). has : ~ ~ ~2(t + t) and ~t )> ~! t ~ where ~ t ~ one has are harmonic oscillator wave functions. In this case In fact in this particular case one expands the potential function V = 2 ~ c~~ q2 t " in all

5 In v2dependence OF CENTRIFUGAL DISTORTION L15 vibrational displacements (including q2). This formula is closely connected with that of reference 14. /2~y~ 3.2 INERTIA TENSOR EXPANDED IN ANHARMONIC ZEROORDER POTENTIAL p ~ ~ 2 B p li2 q2, FUNCTION ~o(p) THE LEADING NON ZERO CONTRIBUTIONS IN EQUATION 4 ARE KEPT. One obtains : This formula is quite similar to that used by Creswell and Mills [15]. 3.3 BOTH APPROXIMATIONS (10) AND (12) ARE USED IN EQUATION 4. this particular case one has the wellknown WilsonHoward formula for a semirigid molecule which is independent on t = V2. Acknowledgments. We are pleased to thank G. Amat, P. R. Bunker, C. CamyPeyret, J.M. Flaud and Yu. S. Makushkin for very helpful discussions. References [1] CAMYPEYRET, C., FLAUD, J. M., Mol. Phys. 32 (1976) [2] FLAUD, J. M., CAMYPEYRET, C., Mol. Phys. 26 (1973) [3] FLAUD, J. M., CAMYPEYRET, C., J. Mol. Spectrosc. 51 (1974) CAMYPEYRET, C., FLAUD, J. M., Spectrochem. Acta 29A (1973) CAMYPEYRET, C., FLAUD, J. M., J. Mol. Spectrosc. 59 (1976) [4] CAMYPEYRET, C., FLAUD, J. M., MAILLARD, J.P., J. Physique Lett. 41 (1980) L23L24. [5] STARIKOV, V. I., TYUTEREV, V1. G., On the theory of vib.rot. spectra of nonrigid molecules (in Russian), Preprint N29, IAO, Tomsk, 1979, 66 pp. STARIKOV, V. I., MAKHANCHEJEV, B. I., TYUTEREV, V1. G., in the book : Spectroscopy of Atmospheric Gases, Nauka, Novosibirsk, 1982, [6] STARIKOV, V. I., TYUTEREV, V1. G., J. Mol. Spectros. 95 (1982) [7] STARIKOV, V. I., TYUTEREV, V1. G., Optika i Spectrosc. 51 (1981) [8] HOUGEN, J. T., BUNKER, P. K., JOHNS, J. W. C., J. Mol. Spectrosc. 34 (1970), [9] HOY, A. R., BUNKER, P. R., J. Mol. Spectrosc. 52 (1974) [10] SØRENSEN, G. O., Topics in Current Chemistry (SpringerVerlag, Berlin) [11] WATSON, J. K. G., J. Chem. Phys., 46 (1967) [12] NIELSEN, H. H., Rev. Mod. Phys. 12 (1940) [13] AMAT, G., NIELSEN, H. H., TARRAGO, G., Rotation Vibration of Polyatomic Molecules (Dekker, New York) [14] MAKUSHKIN, Yu. S., TYUTEREV, V1. G., Phys. Lett. 47A (1974) [15] CRESWELL, R. A., MILLS, I. M., J. Mol. Spectrosc. 52 (1974)