Specmchimica A&, Vol. 49A. NO. 4, pp. 511-525.1993 05&8539/93 s6.oo+o.al Printed in Great Britain @ 1993 Pergamon Press Ltd

Polarized Raman spectra, thermodynamic functions and barriers to internal rotation of 2,3=dimethoxy toluene

0. P. SINGH,* R. P. SINGH AND R. N. SINGH

Department of Physics, Bihar University, Muxaffarpur 842001, Bihar, India

(Received 12 May 1992; in final form and accepted 8 August 1992)

Ah&met-Polarized Raman spectra of 2,3-dimethoxy toluene have been recorded in the region 504000 cm-’ and IR spectra in the region 200-4ooo cm-‘. Au the 63 (4Oa’ + 234 normal modes of vibration have been assigned assuming a C, point group. Consistent assignments for the internal modes of vibration of methyl (CH,) and methoxy (OCH,) groups have been proposed. In addition thermodynamic functions have been computed over the temperature range 100-1500 K on a MIGHTY II computer and barriers to internal rotations for the three methyl (CH3 tops and the two methoxy (OCH,) tops about their respective axes have been determined, using the assigned torsional frequencies and assumed structural parameter for the 2,3dimethoxy toluene. The barrier heights have been found to be greater than 2.5 kcal mol-’ for all five tops.

IN RECENT times toluene and substituted toluenes have become very important on account of their wide range of applications in medicine and industry. The vibrational spectra of toluene or benzene derivatives with methyl (CH,) groups have been studied by several workers [l-13]. Some workers [14-231 studied anisoles or benzene derivatives containing the methoxy (OCH,) group. The molecule 2,3-dimethoxy toluene has been studied by GOEL et al. [23], but some of the fundamental frequencies, and also measurements of the depolarization ratio, were not reported. In addition the thermody- namic functions and barriers to internal rotation of substituent groups have not been calculated. Moreover, there are disagreements in the assignments of ring breathing and trigonal bending a(C-C-C) modes. The 0CH3 group gives rise to two types of internal rotations, one due to OCH, group about C,,,- 0 axis and the other due to the CH, group about O-C,,, axis. Each methyl group also has its own internal and external vibrations. In this way the spectra become complicated and the study very interesting.

The present paper reports the complete vibrational assignments for this molecule (hereafter called 2,3-DMT) along with the depolarization ratio measurements of the Raman lines which have not been reported earlier. In addition, the thermodynamic functions (enthalpy, heat capacity, free energy and entropy) have been computed using the above vibrational frequencies in rigid rotator and harmonic oscillator approximation for one mole of an ideal gas at standard pressure in the temperature range 100-1500 K. The barrier heights for the internal rotation of the three methyl (CH,) groups and the two methoxy (OCH,) groups have been computed. These details are being reported for the first time for the molecule under study.

EXPERIMENTAL

Spectroscopically pure (purity >!W%) chemical 2,3-DMT was obtained from M/s Aldrich Chemicals, U.S.A. and was used as such without further purification. It is a colourless liquid at room temperature.

The Raman spectra for the compound 2,3-DMT were recorded in the liquid phase in the region SO-4lXlO cm-‘, using a Jobin-Yvon Ramanor HG.2S spectrometer. The source of excitation was 488.0 nm radiation from an argon ion laser. Raman spectra have been recorded along with the measurement of depolarization ratios. The IR spectra have been recorded in the liquid phase in the

* Author to whom correspondence should be addressed, at Department of Physics, Y. N. College, Dighwara 841 207, Saran, Bihar, India.

517

518 0. P. SINGH etal.

- AXIS

Fig. 1. 2,3-Dimethoxy toluene. G = Phenyl ring centre; G’ = Centre of mass of the molecules; r(C-C)= 1.4A; r(C-H)= 1.08 A; r(C,+,-O)= 1.37 A; r(O-C,,,,&= 1.47 A; r(C-HtiYJ= 1.09 A; r(C-H,,,& = 1.087 A; LCCC = 120”; LCOC = 120”; LOCH = 109.5”; LCCH(CH3) =

111.1”; angle between ZB axis and y-axis = 41.39”.

region 200-4000 cm-’ on a Pye-Unicam SP 2000 model, double beam grating spectrophotometer using CsI optics. The frequencies reported here are correct to within + 1 cm-’ for sharp bands and + 3 cm-’ for broad bands in both the Raman and IR spectra. A large number of bands has been obtained for the molecule.

RESULTS AND DISCUSSIONS

For the molecule 2,3-DMT, neither microwave nor X-ray diffraction results have previously been reported. In the absence of definite structural parameters, the molecule 2,3-DMT is assumed to a first approximation to belong to the C, point group of symmetry. However, a local C,, point group of symmetry is assumed for the methyl (CH,) group. One of the three hydrogen atoms of the CH3 group is taken to be in the plane of the phenyl ring and the remaining two hydrogen atoms are taken to be symmetrically above (blank lines in Fig. 1) and below (dark lines in Fig. 1) the plane of the phenyl ring. Therefore, the phenyl ring plane is the plane of symmetry. The x-axis has been taken perpendicular to the plane of symmetry of the molecule and the molecule has been assumed to lie in the yz-plane.

The molecule 2,3-DMT contains 23 atoms (N=23) and hence there are 63 (3N-6) normal modes of vibration which may be classified as 40 planar (a’ species) and 23 non- planar (a” species). Their groupwise distribution is as follows:

(i) phenyl ring mode-2la’ + 9~” = 30 (ii) CH, group mode-%’ + 4a” = 9

(iii) 0CH3 group mode-2(7a’ + 5~“) = 24. The vibrational assignments for the observed frequencies have been proposed on the

basis of magnitudes and relative intensities of the frequencies observed in the Raman and IR specta as well as the depolarization ratio measurements. In addition assistance has also been taken from the vibrational assignments proposed by earlier workers [l-26].

2,3-Dimethoxy toluene 519

The proposed assignments along with depolarization ratios have been given in Table 1. The calculated values of principal moments of inertia (IA, ZB and Zc), reduced moments of inertia (ZJ and barrier heights (V) are given in Table 2 while the thermodynamic functions are given in Table 3. Figure 1 shows the assumed structural model, principal axes, the different five top axes and their orientations relating to the molecule hxed Cartesian coordinate axes for the molecule, while Fig. 2 shows the variation of enthalpy [Ho - ZQT], heat capacity [Cj], free [Go - Ey T] and entropy [So] with absolute temperature (T).

Vibrational spectra

The vibratonal frequencies arising due to the phenyl ring modes are well established and therefore are not discussed in detail. For the assignments of the phenyl ring mode, Table 1 is self-explanatory. However, some of the phenyl ring modes are discussed below.

The parent molecule benzene has two doubly degenerate modes-e,, (1585 cm-‘) and e1,(1485 cm-‘)--and two non-degenerate modes b,(1310 cm-‘) and a1,(992 cm-‘) due

Table 1. Vibrational frequencies (cm-‘) and assignments for 2,3-dimethoxy toluene

Raman Rel. int. Depol. ratio IR Rel. int. Species Proposed assignments Wilson no.

1615 1600 14% 1470 1465

1455 1445 1425 1410

1355 1340 1320 1305 1285 1245 1240

W

mw mw mw sh mw S

vs

S

mw mw S

S

W

mw W

W

W

mw mw vw sh vw mw W

W

ms sh vs W

W

0.67 0.68 0.46 0.58

0.67 0.90 0.94 0.90 0.81 0.64 0.70 P+

0.40 0.51 0.54 0.50 0.50 0.76 0.83 0.81

0.52 0.51 P+

0.50 0.56

0.49 0.61 0.55

2980

2875 72

2750 32 2600 34 2410 31 2195 32 2005 36 1975 33 1920 33 1840 34 1730 33 1620 70 1608 87 1505 99 1465b 83

1462 1450 1445

1390

1325 1299 1287

1245

48

73 87 87

83 85 85

41

92 100 100

99

a’ a’ a’ a’ a’ a’ d

a” a” a’ a’

I-k A’ A’ A’ A’ A’ A’ A’ A” a’ a’ a’ a’ a’ d a” a” a’ a’ a’ a’ a’ a’ a’ a’ a’ a’

v(C-H) v(C-H) v(C-H) MCH3)

K&H,)

dCH3)

%d’=3)

Fermi resonance dCH3)

%s(~3)

c(CH3)

c(CH3)

dCH3)

1450+1299;1245+1505 1299x2 1299+1105;1620+795 1325 + 875 1608 + 400; 2875 - 875 1105 + 875; 1052 + 920 1390 + 530 920x2 1027+702 v(C-C) v(C-C) v(C-C) v(C-C)

UCH3)

4dCH3)

4CH3)

UCH3)

hACH3)

UCH3)

UCH3)

4@43)

UCH3)

~&yi-0)

NC-H) v(C-C) Kekuk mode +uyl-o) v(C-CH,)

2 200 206

8a 8b

190 196

13 3

14 7a 76

U(A) 49:4-E continued on next page

520 0. P. SINGH et al.

Table l-continued

Raman Rel. int. Depol. ratio IR Rel. int. Species Proposed assignments Wilson no.

1200 W

1180 W

1105 ms 1060 vw 1040 W

1025 s

1020 mw 1005 mw 975 VW

920 W

970 sh 880 W

840 W

817 S

760 mw 695 S

565 s

560 s

530 vs

495 S

490 S

405 mw 385 s 355 S

310 sh 270 mw 255 mw 250 mw 230 S

230 S

220 mw 100 S

66 2

0.53 0.52 0.62 0.79 0.44 0.20

dp+ 0.84 0.85 0.79

0.48 0.47 0.81

0.54 0.95 0.65 0.67 0.55 0.52 0.76 0.83 0.72 P+

0.80 0.86 0.78

dp+ dp+ dp+ dp+

1194 77

1105 100 1052 55

1027 99

920 49

875 31

820 67 795 34 762 92 702 67 560b 28

530 33 490b 31

400 30 390 35 360 29 310 31 280 28 255b 36

230 36 230 36 220 28

a’ a’ a’ a” a’ a’ a” a” a” a” a’ a’ a’ a” A’ a’ a” a’ a’ a’ a’ a” a” a’ a’ a’ a” a” a” a” a” a”

B (C-W v(caIky,-o) B (C-H) PI v(cdky,-o)

Ring breathing

PI

;;C-H)

Y (C-H) PII PII PII Y (C-H) 490 + 310 o (C-C-C) trigonal bending rp (C-C-C-C)

B (CNO) B (r&,-O) a (C-C-C) a(C-C-C) qJ (C-C-C-C) f@ (C-C-C-C)

B (C-CH,) C-O-C bending C-O-C bending

Y (Car,,-0) Y (C,l-0) Y (C-CH,) r (CH,) r (CH,) r (CH3)

r (CH3)

dCH3)

9a

186

1

10a lob

11

12 4

18a 96 6a 66

lti 16b 15

17a 176 5

Y: stretching; Y,: symmetric stretching; Y,: asymmetric stretching; 6,: symmetric deformation; 6,: antisym- metric deformation; Q: angle bending; fi: planar bending; y: non-planar bending; pl: perpendicular rocking; 4,: parallel rocking; #: ring torsion; t: torsion; vw: very weak; w: weak; mw: medium weak; ms: medium strong; s: strong; vs: very strong.

P+ and dp+ are used for those Raman lines for which depolarization ratio could not be determined precisely. a: taken from another reference.

to skeleton stretching of C-C bonds. In all substituted benzenes, four bands are usually observed in the region 1400-1650 cm-’ due to the splitting of the two doubly degenerate frequencies into four totally symmetric components. The actual positions are determined not so much by the nature of the substituents but by the form of substitution around the ring, suggested by BELLAMY [U]. Four bands observed at 1615, 1600, 1495 and

Table 2. Principal moments of inertia, reduced moments of inertia (10-39gcm2) and barrier heights (kcal mol-‘)

Principal moments of inertia Top axes

Reduced moments Symmetry of inertia (I,) number (n)

Barrier heights 20 r,v*

v=-_x- 7 2.8n2

IA= 56.85803 x 1O-39 1st top - T, axis 0.5231109 x 1O-39 3 3.1375 IInd top - 7’r axis 0.5231109 x 1O-39 3 3.1375

la = 76.25391 x 1O-39 IIIrd top - T3 axis 0.5114385 x 1O-3g 3 2.8065 zc= 131.59707 x 10-39 IVth top - T4 axis 3.8494308 x 1O-39 2 9.8199

Vth top - TS axis 3.7857802 x 1O-39 2 4.2068

2,3-Dimethoxy toluene 521

1470cm-* have been assigned to these two degenerate modes of benzene, and finds support from the assignments of earlier workers 19, 10, 151.

‘Ihe KekulC mode gives rise to its characteristic frequency in the region l200-1400 cm-’ for substituted benzenes and is easily assigned on account of its nearly pure (C-C) stretching nature. We get a well polarized and very strong band at 1285 cm-’ and it is assigned as a KekulC mode. This assignment is in agreement with earlier workers [15,16].

The ring breathing mode a,,(992 cm-‘) and a(C-C-C) trigonal bending non- degenerate mode 61,(1010 cm-‘) of benzene are intimately related and are affected by substitution. According to BAILEY et al. [28] the mono and 1,3,5trisubstituted benzenes have two intense Raman lines in the region of about 1000 cm-’ and the line with a very low depolarization ratio is assigned to the ring breathing mode (l), whereas the line with a higher depolarization ratio is assigned to the trigonal bending mode (12). VARSANYI [29] has suggested for vl a different frequency region based on the nature [light mass < 25 amu and heavy mass > 25 amu] and the number of substituents irrespective of the relative intensity and Raman depolarization ratio. VARSANYI [29] and MILLER [30] also suggested that for the meta-isomer the v 12 mode should be assigned to a strong and polarized Raman line at 1OOOrt lOcm_‘. However, many workers [9, 16, 25, 261 have assigned v1 in substituted benzenes to a frequency having characteristics to the 992 cm-’ Raman line of benzene. We also believe the ring breathing mode vl, being derived from the most symmetric species alg of benzene, should retain its characteristics in the benzene derivatives and should be assigned to a strong and highly polarized Raman line. In the present case the strong band observed at 1025 cm-’ having the lowest depolarization ratio (0.20) has been assigned as a ring breathing mode. The earlier workers GOEL et al. [23] assigned this mode as Raman 685 cm-’ (ws P) and the corresponding IR at 688 cm-’ (ws). In this respect we disagree with the earlier assignment. Although we have observed a band at 695 cm-’ with strong Raman intensity its depolarization ratio is very high (0.95), and hence this depolarized line has been assigned as ring torsion @(C-C-C-C) mode 4.

The trigonal a (C-C-C) bending mode of benzene is very sensitive to the position and nature of the substituent and is drastically reduced in magnitude due to its interactions with other modes. Due to the presence of the three substituents at three different positions the reduction is large. The band observed at 760 cm-’ with mw Raman intensity and higher depolarization ratio (0.54) has been assigned to trigonal bending mode a(C-C-C). The earlier worker has assigned this mode at 1008 cm-‘. Our assigned band frequency is present in the region as suggested by VARSANYI [29].

Table 3. Thermodynamic functions in (cal mol-’ K-l)

Enthalpy Free energy

Temperature (Ho-e) Heat capacity -(Go-EO, Entropy T(K) T ICZI T ISOl

100 11.6439 17.7539 56.4040 68.0479 200 18.1257 31.1069 66.4484 84.5741 300 24.5658 43.8328 75.0174 99.5832 400 30.9573 56.2606 82.9621 113.9194 500 37.1409 67.1876 90.5401 127.6810 600 42.9331 76.3135 97.8305 140.7636 700 48.2586 83.8771 104.8547 153.1133 800 53.1171 90.1903 111.6209 164.7380 900 57.5398 95.5041 118.1367 175.6765

1000 61.5674 100.0055 124.4108 185.9783 1100 65.2406 103.8368 130.4537 195.6943 1200 68.5%9 107.1105 136.2765 204.8733 1300 71.6701 109.9181 141.8903 213.5605 1400 74.4906 112.3349 147.3063 221.7969 1500 77.Ofw 114.4231 152.5353 229.6202

522 0. P. SINGH et al.

230-

200 -

I 1 I I I I I I 1

0 200 600 600 600 1000 1200 1400 1600 TEMPERATURE,eK

Fig. 2. Variation of thermodynamic function with temperature (T).

The survey of literature reveals that a band around 12OOcm-’ appears due to the valance oscillation in toluene and substituted toluenes. The polarized band observed at 1240 cm-’ has been assigned to the (C-CHS stretching mode for the molecule 2,3-DMT. The in-plane bending and out-of-plane bending modes of C-CH3 have been assigned at 385 and 250 cm-‘, respectively, and agree with the assignments made by PRABAKARAN and MOHAN [lo] based on normal coordinate analysis. The reported papers indicate that the stretching C,,,- 0 modes give rise to its characteristic absorption centred around 1250 cm-‘. Due to the presence of two methoxy (OCH,) groups, this range may be widened as is clear from the assignments of MARIIT et al. [15]. We have assigned the two C,,,-0 stretching modes at 1320 and 1265 cm-‘. These assignments are in agreement with OWEN and HESTER [14],G0~~etaZ. [~~]~~~VENKATACHARYULIJ~~ al. [17].Mostof the phenyl assigned modes have been given in Table 1 and find support from the assignments of earlier workers.

CHJ0CH3 group vibrations

Under the point group C,,, the CH3 group gives rise to three non-degenerate modes (symmetric stretching, symmetric deformation and torsion) and three degenerate modes (anti-symmetric stretching, anti-symmetric deformation and rocking). On reduction of symmetry from C,, to C, each of the three degenerate modes splits up into two components. One component of each belongs to the symmetric species (a’), and the other one to the antisymmetric species (a”). In this way the methyl (CH,) group has its total nine (5~’ + 4~“) normal modes of vibrations.

The spectra of the molecules having a methoxy (OCH3 group should comprise the additional three modes for each methoxy (OCH3) group along with all the above mentioned nine modes of the methyl (CH3) group. The three additional modes are-one v(O-CHJ (i.e. C,,- 0 stretching), one C-O-C bending and one torsion mode for each methoxy (OCH3) group. Here we have two methoxy groups and one methyl group,

2,3-Dimethoxy toluene 523

hence the molecule 2,3-DMT should have a total 33(19u’+ Mu”) modes of group vibrations, i.e. internal modes of vibration of groups.

The asymmetric stretching vibrations have been assigned in the region 2900-3ooO cm-‘, while symmetric vibrations in the region 2MO-2900 cm-’ are given in Table 1. The relative Raman intensity of the 2935 cm-’ is very high in comparison with the other modes and its depolarization ratio is also high, hence it may be explained as a Fermi resonance line due to 2940 and 2930 cm-‘.

The CH3 symmetric deformation and CH3 rocking modes are sensitive to the electro- negativity of the substituent (attached) atoms. Hence the CH3 rocking vibration is coupled with the C-CH, symmetric stretching vibrations. The frequencies of the perpendicular rocking vibration are always found to be greater than parallel rocking vibrations. The assigned frequencies corresponding to these modes have been given in Table 1.

The survey of literature reveals that the Y(O-CH3), i.e. C,,,,-0 stretching vibration, lies within the range 1000-1050 cm-’ as suggested by OWEN and HESER [14]. But due to the presence of two methoxy (OCHJ groups, the bands observed at 1180 and 1040 cm-’ have been assigned to this mode. Similarly C-O-C bending modes have been assigned at 355 and 31Ocm-‘. The above assignments find support from the assignments of VENKATACHARYULU et al. [17], GOEL et al. [23] and MARJIT et al. [15].

The torsional frequency is a very important mode because it plays a vital role in the calculation of the barrier to internal rotation. We can obtain only two bands at 230 and 220 cm-‘. The depolarization ratio for these bands could not be measured. These bands have been assigned as a torsion mode z(CH,) of the methyl group.

The torsional band frequencies due to OCH3 group have not been observed by us satisfactorily due to the lack of IR spectra below 200 cm-‘. However, we can obtain only one band at 100 cm-’ in the Raman spectra of the molecule 2,3-DMT. We have taken 66 cm-’ as the second torsion mode of methoxy (OCH,) from the work of GOEL et al. [23]. The magnitude of torsional frequency due to the T,top is expected to have higher magnitude than that due to the T,-top. The other assignments find support from earlier works and are given in Table 1.

Some of the overtones and combination bands have also been observed and are given in Table 1.

Thermodynamic functions and barrier to internal rotation

Due to absence of microwave or X-ray diffraction data for the structure of the molecule 2,3-DMT, the assumed structural parameters have been taken from the works of OWEN and HESTER et al. [14] and DONGRE et al. [13], and are also given in Fig. 1.

With the above mentioned assumed parameters the principal moments of inertia IA, ZB and Zc were calculated, employing the method suggested by HIRSHFELDER [31], and their orientations with respect to the molecule fixed Cartesian co-ordinate axes were deter- mined. From the calcuated values of the principal moments of inertia and their orientations with respect to the local top axes (as shown in Fig. l), the reduced moments of inertia (I,) for the three CH3 tops (T, to T3) and two OCH, tops (T4 and Ts) were determined, employing the methods suggested by PITZER and GWINN [32].

The barriers to internal rotation for all the five tops have been calculated following the method of DURIG et al. [33] as follows:

20 z*xv* V,‘TXS

where n = symmetry number, I,= reduced moment of inertia and v= torsional fre- quency. In the present case the symmetry number n = 3 for the methyl (CH,) group and n = 2 for the methoxy (OCH3) group. The calculated values of the principal moments of inertia (I,, ZB and Zc), reduced moments of inertia (Zr) and barrier to internal rotation (V) for the different five tops are given in Table 2. It is clear from Table 2 that for all the tops the barrier heights are greater than 2.5 kcal mol-‘. Hence it seems reasonable to treat the

524 0. P. SINGH et al.

torsional frequencies like the other fundamental frequencies for the computation of the vibrational contribution to the thermodynamic functions as suggested by DURIG et al.

P31- Using the above assigned fundamental frequencies, the thermodynamic functions were

calculated employing the standard expressions given by COLTHUP et al. [34] and

HERZBERG [35]. The translational, rotational and vibrational contibutions to the thermo-

dynamic functions were computed separately on a MIGHTY II computer equipped with a UNIX compiler in the temperature range 100-1500 K by using the 63 fundamental frequencies, assuming rigid rotator and harmonic oscillator approximation for one mole of an ideal gas at standard pressure; however, in Table 3 only the sum is given. The separate calculations show that the translational and rotational contributions to enthalpy and heat capacity are temperature independent while the vibrational contribution increases gradually with temperature. Moreover, the translational and rotational contri- butions to the free energy and entropy dominate over those of the vibrational contribu- tion at lower temperatures and at higher temperatures, the three contributions become comparable for free energy and the vibrational contribution dominates over the other two contributions for entropy. The variation of thermodynamic functions with tempera- ture have been shown in Fig. 2. The nature of variation agrees with YADAV et al. [25].

CONCLUSIONS

It is interesting to note that most of the fundamental vibrations assigned in the Raman spectra agree favourably with the JR frequencies of the compound. The values of barrier to internal rotation and thermodynamic functions are in a comparable range with the values of earlier workers.

AcknowIedgemen&--The authors are grateful to Prof. G. Thyagarajan, Head of the Regional Sophisticated Instrumentation Centre, I.I.T. Powai, Bombay, India for providing facilities for recording IR and Raman spectra of the molecule at his centre. We are very grateful to Dr R. A. Yadav of the Department of Physics (Laser Spectroscopy Laboratory), Banaras Hindu University, Varanasi for giving valuable suggestions and encouragement in the course of this work.

REFERENCES

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2,3-Dimethoxy toluene 525

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