Pc2h = 22A~-}-lTB~+18Au + 21B~.

Transcription

1 INTERRETATION OF THE VIBRATIONAL SECTRA OF TERT-BUTYL AND DIMETHYLETHYNILMETHYL EROXIDES I.. Zyat'kov, G, A. itsevich, A.. Yuvchenko, Yu. L. Ol'dekop, V. I. Gogolinskii, V. L. Antonovskii, and D. I. Sagaidak UDC The IR and Raman spectra of the peroxides of the R(CH3)2"COOC(CH~)2R (R = CH 3 (I) and C~CH (II)) alkyls were ohtained. The synthesis of the latter was described in [i]. The IR and Raman spectra of peroxide II were obtained and interpreted for the first time. The vibrational spectra of peroxide I were studied in [2-4] but the attribution of individual bands and lines is contradictory. The complete interpretation of the IR and Raman spectra of this compound proposed in [5] is based on the assumption of the realization of C 2 symmetry for molecule I (the dihedral angle C-<)-O-C ( was assumed to be 125~ According to x-ray analysis ~ = 164 ~ [6]. The increase of the dihedral angle by 40 ~ relative to the methyl peroxide molecule [7] must be accompanied by a lowering of the trans barrier, which is very low even for the (CH30)2 molecule (i kj/mole). Since the cis barrier for compound I is considerably larger, then at room temperature the majority of molecules must exist in the trans conformation or a conformation close to it. Consequently, it is more acurate to interpret the IR and Raman spectra of molecule I using the C2h symmetry. An important property of the vibrational spectra of molecules with this symmetry is the satisfaction of the rule of mutual exclusion. It is correct for the majority of experimental frequencies of peroxide I. According to [5] vibrations at 514, 1020, and 1240 cm -I appear in both vibrational spectra. In our experiment the 514 cm -I is absent in the IR spectrum. The 1020 and 1240 cm -I vibrations appear in the IR and Raman spectra as a consequence of the distortion of the C2h symmetry. In the present work we interpret the vibrational spectra of peroxides I and II. To achieve this we used literature data, performed polarization measurements in the Raman spectra, obtained the IR and Raman spectra of deuterated peroxide I (I--dis), and calculated the spectra of the normal modes of compounds I and I-d~8. For compound I-dzs we did not perform a complete interpretation of the IR and Raman spectra, because in the sample studied there was up to 20% ether CH3CH2OC(CH3)3, the medium where the synthesis was carried out, and up to 4% peroxide I. To determine the bands and lines that belong to compound I-d18 we used the IR and Raman spectra of compounds I and CH3CH2OC(CH3)3. The IR absorption spectra of the compounds studied were recorded on a Specord 75-IR with a slit spectral width of 3 cm -I. The samples were placed as a thin layer between KBr windows. The Raman spectra were recorded on a Spex-Ramalog instrument using a slit spectral width of 2 cm -I The samples were excited using a cm -~, 0.2 W argon laser. The accuracy of calculation of the wave numbers and the polarization measurements were controlled using a CCI 4 sample. Calculation of the normal modes spectra was carried out using a program described in [8]. In the calculation we used the force constants of isobutane [9] and methyl peroxide [i0], and also introduced the interaction constants of the C-O bonds and C-O-O angles ( and 0.25"106 cm -2, respectively). Using the C2h symmetry, molecules I and I--d18 have the following vibrations with the specified symmetries: c2h = 22A~-}-lTB~ B~. The and vibrations can appear only in the, and the lines due to the vibrations are polarized while the ones due to the vibrations are depolarized. In the IR absorption spectrum A u and B u vibrations can be observed. The O-O valence vibration in Translated from Zhurnal rikladnoi Spektroskopii, Vol. 49, No. 2, pp , gust, Original article submitted June 26, /88/ lenum ublishing Corporation 835

2 molecule I is totally symmetric and must appear only in the in the form of strong, polarized line. In the interval where it is expected to appear, cm -i, three lines at 775, 862, and 904 cm -i have these properties. The 775 cm -i line has spectral properties similar to the 799 cm -i line in the isobutane [9] and, consequently, is due to the totally symmetric C-C valence vibration in the tertiary fragment. In [4] it is assumed that the 862 and 904 cm -i lines are due to strongly mixed vibrations of the O-O and C-O bonds. However, according to [ii], in the vibrational spectra of tert-butyl hydroperoxide the O-O valence vibration appears at frequency 843 cm -i The frequency of the C-C valence vibration is 884 cm -i. The main contribution of the C-O valence vibration is in the 1246 cm -i frequency. The interaction of two C-O bonds in peroxide I leads to their splitting into symmetric and antisymmetric vibrations with the latter having a higher frequency. Since the nature of their surrounding in tert-butyl hydroperoxide and peroxide I is practically the same, then the splitting must be approximately symmetrical with respect to 1246 cm -i, the frequency of the vibration of the noninteracting C-O bond. If it is assumed that the symmetric vibration of the C-O bond corresponds to the 904 cm -i line, then the splitting will be unjustifiably large: almost 600 cm -i. Obviously, it is more suitable to assume that the 904 cm -i line is similar to the 884 cm -i line in the spectrum of hydroperoxide and is due to the symmetric valence vibration of the C-C bond, and that the 862 cm -i line is due to the valence vibration of the peroxide bond. The calculation data of the normal modes spectra of molecule I are in agreement with the experimental attribution. The calculated frequencies of totally symmetric valence vibrations of the C-C bond are 754 and 945 cm -i, and for the valence vibrations of the 0-O and C-O bonds they are 834, 1322, and 1366 cm -i, respectively. The contribution of v0-0 to the 862 cm -i frequency reaches 57%. The contribution of the deformation vibration of the 0-O bonds and the valence vibration of the C-C bonds is 18 and 14%. The contribution of the valence vibrations of the C-O bonds is only 10%. Thus, because the total contribution of the valence and deformation vibrations of the peroxide bond is 75%, it is possible to assume that the 862 cm -i vibration is localized on the peroxide bond. According to the calculation, the rocking vibrations localized on the C-H bonds are in the cm -i interval where their contribution reaches %. The contribution of these vibrations to the 927 cm -i frequency is 40%, and the contribution of the valence vibrations of the C-O bonds is 27%. In the cm -i spectral interval the rocking vibrations of the C-H bonds are mixed in parity with the valence vibrations of the C-C bonds. According to the data in [i0], the splitting between the symmetric and antisymmetric C-O vibrations in the methyl peroxide molecule is I00 cm -i. Using this information, and also assuming that the frequency of the noninteracting C-O vibration is approximately 1246 cm -i, we can attribute to the symmetric vibration of the C-O bonds in peroxide I the 1209 cm -i line, and attribute to the antisymmetric vibration the 1243 cm -i band. The contribution of the valence vibrations of the C-O bonds to these frequencies is 40 and 42%, and the contribution of the rocking vibrations of the C-H bonds is 28 and 24%. The symmetric vibration of the C-O-O angle is attributed to the strong 243 cm -i line in the. Six valence vibrations of the C-C bonds in peroxide I are presented by the type of symmetry as follows: 2 + i + IA u + 2B u. The vibrations are related to the 775 and 904 cm -i lines. Calculation shows that the first vibration is localized (the contribution of nc- C reaches 70%). For the second vibration the contribution of vc-c is 45% and of ~C-C-H is 30%. Also there is a small contribution from the O-O valence vibration, 15%. The 913 cm -z depolarized line (the calculated frequency is g34 cm -i) is attributed to the antisymmetric (relative to the C z axis) C-Cvalence vibration, and the 749 cm -i absorption band (the calculated frequency is 722 cm -i) is attributed to the antisymmetric (relative to both symmetry elements) C-C vibration, The 514 cm -i line (the calculated value is 551 cm -i) is attributed to the O-C-C totally symmetric vibration. Its high intensity is due to the considerable contribution (up to 25%) of the symmetric vibration of the C-O-O angles. The four C-C deformation vibrations include one of each vibration type. Since these vibrations appear below 450 cm -i, then in the IR spectrum which is recorded up to 400 cm -i we attribute one 428 cm -i band. To the totally symmetric deformation vibrations of these bonds we attribute the 290 cm -i line. The depolarized 408 cm -z line is attributed to the same vibrations with a symmetry. Their calculated frequencies are 414, 286, and 382 cm -i. For the deteurium analogs of molecule I the calculated ~0-0 frequency is 916 cm -z. In the of the I-dis compound in the cm -i interval there is only one strong line at 875 cm -i This line is not similar to the 905 cm -z in the of 836

3 TABLE I. Interpretation of the Vibrational Spectra of Molecule I a, L Av Calculated K requency, cm -x ED 50 $ :~oo, 46rco 43Zoo, 57Tco o 156occ 62Zc o, 14~ooc 30600o 226occ 4 90TCH3 92TCH ~ 90TCH ~ 92TCH 3 92TCH3 92XCH~ 566occ, 8~ccc, 4Voo 626cc C, 236occ 666cc c, 106oc c 726cco 66occ 79, 46occ 446cc o, cc o, 228ccc 596cc o, cc o, 236oo C, occ, 156coc 88~cc 66Vcc, 186oo C, 4Vc o 57Voo, 14Vcc, 186ooc 63VCC, 316CC H 62Vcc, 326CCH 40VCC, 336CC H, 12VOO 48VCC, 36 38VCO, 476CCH 356CC H, 28VCO, 10VCC 928cc n 926cc H 466cc H, 30Vcc 466cc H, 30Vcc 526CC H, 3Ovcc 496cc H, 28Vcc 42Vco, 246cc H, 8Vcc 44Vco, 126cc H, 6Vcc 50~CC H, 45~HC H 50~CC H, 45~HC H 50C~CC H, 45(~HC H 50(~CC H, 45 HC H 506CC H, 45t~HC H 506CCH, 456Hr H 986HCII 986rich II~ sv2crrum! v, 1" p Icm, /cm' '* C 15C 9O lh. 5O ,75 sh. 0,45 8 0,5 6 0,75 sh. 2 0,6 1 0, , O 2 3 sh. sh. 3O

4 TABLE 1 (continued) At, B~ A,, Ae A,~ A. A~t Ilated- Calcu- Ifrequen- " cm -I ED ~Y ncH 985HCH 986~cH 986HC H 99Vc H 99vC H 99VcH 99Vcr-~ 99'VcH 99Vci~ 99VcH 99Vc H 99VcH 99X:cH 99Vc H 99vCH I v, 1" 0 cm sh O, 1 I [ 0, , i0,75 J c~ A i t I I 95 *The intensity in the Raman spectra is in relative units at the maximum of the spectra line. %The band intensities in the IR spectra are in % of absorption. %The numbers are the % of the contribution in the potential energy distribution. compound I, because on the one hand the calculation predicts the lowering of the frequency of the C-C symmetric valence vibrations attributed to the 905 cm -I line by more than 150 cm -I in molecule I-dls, and on the other hand, according to calculation, a similar shift must take place for line 913 cm -I attributed to a ~C-C vibration in molecule I. In the era -I interval of the of molecule I-dzs there are no depolarized lines, while there are lines, including depolarized ones, in the cm -I interval with no analogs in the of molecule I. Thus, the 875 cm -I line must be attributed to ~O-O in molecule I-dls which supports the calculation prediction and the attributions of bands and lines in the IR and Raman spectra. For the peroxide molecule II it is possible in principle to have several conformers related to the rotation relative to the C-O bonds. For conformers with symmetry C2h the CECH bonds in both tertiary fragments are in the trans position relative to the O-~ bond. If the methyl groups are in the trans position relative to the peroxide bond, then molecule II will have a C 2 or C i symmetry. In the first case both C~CH bonds lie on the same side of the C-O-O-C surface, and in the second case they are on different sides. Finally, a geometry with a C l symmetry is achieved when the CECH and O-0 bonds are in trans position in one of the tertiary fragments and the O-O and C-CH 3 in another. Thus, the total number of possible conformers for molecule II is large. Analysis of the IR and Raman spectra of peroxide II makes it possible to confirm that in the sample there are conformers with a C I 838

5 TABLE 2. cule II v, am -1 I Interpretation of the Vibrational Spectra of Mole- p! sp ec R tr_um icm-t ' A ttribution v, / p am -1 IRspectrum d- v 1 1 Attribution O O t 851 9OO 80 TCH3 TCHS 6coo 6coo 6occ 6occ ~C~C--C 6C~C--C 5occ 8occ 5occ 6occ ~occ 6c~c-H VCC 'VCC VCC %'O0 ~'oo ~;CC VCC dp loll t , lo00 25 p p dp p dp dp J00 dp dp i i 2C 209C sh i J 120 p ~ dp K %'CC 6CCH ~cch VCO ~;CO %:CO 6CCH ~HCH ~C--=C %'C~- C VC H 'VCH %'CH ~ch 'VCH %'CH or C 2 symmetry, because the principle of mutual exclusion is not satisfied for most of the molecular vibrations. In addition, the number of lines, 45, in the of compound II is considerably greater than the theoretically allowed number of vibrations, 36, active in the Raman spectra when the C2h and C i symmetries are realized. On the other hand, the number of lines in the IR and Raman spectra is not greater than the theoretical number of vibrations for one conformer with symmetry C I. Since in the IR and Raman spectra the lines attributed to the C~C bond have a doublet form, then it is possible to confirm that in the sample there a minimum of two conformers with at least one of them having a C 2 or C I symmetry. The valence vibration of the peroxide bond in molecule II must be manifested in the form of strong polarized lines in the in the cm -l interval. The 861 and 900 cm -I lines satisfy these requirements. Obviously, they are analogs of lines 860 and 905 cm -z in the of compound I. Hence line 861 cm -I can be attributed to vibration v0_ 0 and line 900 cm -I to vibration vc-c" A reasonably strong shoulder near line 861 cm -I with a maximum near 851 cm -I can be attributed to similar vibrations in a different conformer. Since analogs of lines 861 and 851 cm -I lines are absent in the IR absorption spectra, then the peroxide chain must preserve a C2h local symmetry. In the cm -~ spectral interval there are C-H rocking vibrations and C-C and C-O valence vibrations. The C-O antisymmetric vibration must appear, taking into account the realization of local symmetry C2h for the peroxide chain, at higher frequency than the s3nr~etric vibration. The frequency of the noninteracting C-O vibration can be assumed to be approximately equal to the frequency of the vibration of this bond in dimethylethynilmethyl hydroperoxide at 1235 cm -~ [Ii]. This supports the attribution to the antisymmetric C-O valence 839

6 vibration of one of two bands at 1240 and 1225 cm -i, or both if they belong to different conformers, because they are very strong in the IR spectrum. The 1201 cm -i line should be attributed to the symmetric ~C-O vibration. Lines in the cm -i spectral interval are attributed to the rocking vibrations of the C-Hbonds in the methyl groups. To the C-C vibrations in the tert-amyl fragment we attribute two lines in the cm -i and lines 900 and 940 cm -i Below 600 cm -i lie the rocking and deformation vibrations of the carbon skeleton of the molecule. Their total number for any of the conformers studied should be greater than ten. In the IR and Raman spectra of peroxide II at least 12 bands must be attributed to these vibrations, which confirms again the existence of conformers. The 277 and 250 cm -i lines are attributed to 6C_O_O, and the 167 and 178 cm -i lines are attributed to the torsional vibrations of the methyl groups. The interpretation of the vibrational spectra of compounds I and II is given in Tables 1 and 2. Thus, it is established that the valence vibration of the peroxide bond in the case of tert-alkyl peroxides appears near'860 cm -i with high relative intensity in the. In the IR absorption spectrum this vibration is not observed. According to the calculation data, the total contribution of the valence and deformation vibrations of the peroxide bond reaches 75% and, consequently, it can be assumed characteristic for tert-alkyl peroxides. It is shown that for molecule II there are several conformers related to the rotation of the tertiary fragment around the C-O bond. LITERATURE CITED I. A.. Yuvchenko, K. L. Moiseichuk, E. A. Dikusar, and Yu. A. Ol'dekop, Vestsi, Akad. BSSR, Ser, Khim. Navukj No, 2, (1985), 2. H. R. Williams and H. R. Mosher, Anal. Chem., 27, No. 4, (1955). 3. A. R. hilpotts and W. Train, Anal. Chem., 24, No. 4, (1952). 4. H. A. Szymanski, rogress in Infrared Spectroscopy, ~, No. i, (1966). 5. D. C. McKean, J. L. Duncan, and R. K. M. Hay, Spectrochim. Acta, A23, No. 2, (1967). 6. Yu. L. Slovokhotov, T. V. Timofeeva, M. Yu. Antipin, and Yu. T. Struchkov, J. Mol. Struct., 112, (1984). 7. B. Haas and H. Oberhammer, J. Am. Chem. Soc., 106, No. 20, (1984). 8. V.A. Dement'ev, V. I. Smirnov, and L. A. Gribov, FORTRAN rograms for Calculation of Molecular Vibrations, deposited in VINITI on October ii, 1976, No , Moscow (1976). 9. B. Schreder and J. acansky, J. hys. Chem., 8_88, No. 21, (1984). i0. M. E. Bell twill and J. Loane, Spectrochim. Acta, A28, No. 8, (1972). 11. I,. Zyat'kov, Yu, A. Ol'dekop, A,. Yuvchenko, et al., Zh. rikl. Spektrosk., 48, No. 4, (1988). 840