Free jet absorption millimeter wave spectrum of benzene sulphonyl chloride

15 September 1995

CHEMICAL PHYSICS LETTERS

ELSEVIER

Chemical Physics Letters 243 (1995) 302-307

Free jet absorption millimeter wave spectrum of benzene sulphonyl chloride Walther Caminati *, Assimo Maris, Aldo Millemaggi, Paolo G. Favero Dipartimento di Chimica 'G. Ciamician' dell'Universit~t, Via Selmi 2, 40126 Bologna, Italy Received 12 June 1995; in final form 10 July 1995

Abstract

The free jet millimeter wave spectrum of benzene sulphonyl chloride has been investigated in the 60-66 GHz frequency range. The rotational spectra of the 35C1 and 37C1 isotopomers have been assigned, allowing the determination of the equilibrium configuration: the C1 atom is perpendicular to the benzene ring.

The molecular conformation of benzene sulphonyl chloride (BSC, see Fig. 1) has been the object of several experimental and theoretical investigations [1-4]. By electron diffraction Brunvoll and Hargittai suggested the dihedral angle C1S-CIC 2 (~-) to be about 75 ° [1]. Boggia et al. recorded the low-resolution microwave spectrum (LRMW) and concluded, also on the basis of C N D O / 2 calculations [2,3], that the CI atom was in the plane of the benzene ring (r = 0°). Later Van Eijck et al. showed that it would not have been possible, on the basis of LRMW spectroscopy and C N D O / 2 calculations to discriminate between the ~-= 0 or 90 ° conformations. The assignment of the high-resolution MW spec35 37 tra of the C1 and CI isotopomers would have given a precise answer to this problem. We tried several years ago to assign the high-resolution MW spectrum of BSC, but without success: the higher

* Corresponding author.

molecular weight of the molecule and the large amplitude motion (the SO2C1 group torsion) prevented the assignment of single rotational lines with conventional MW spectroscopy. Since a free jet millimeter wave spectrometer is now available, we decided to examine this problem with this valuable technique. A sample of BSC was purchased from Aldrich and used without further purification. It is liquid at room temperature but it has a low vapour pressure (a

CI

Fig. 1. Diagram of the geometry of BSC with the relevant angles and atom numbering.

0009-2614/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 0 9 - 2 6 1 4 ( 9 5 ) 0 0 8 1 0 - 1

W. Caminati et al. / Chemical Physics Letters 243 (1995) 302-307

303

Table 1 Experimental transition frequencies of BSC (MHz)

J'(K~, K~) ~ J"(K~, K~) 16(16)-15(15) a 17(15)-16(14) a 17(16)-16(15) a 17(17)-16(16) a 18(15)-17(14) a 18(16)-17(15) a 18(17)-17(16) ~ 19(14)-18(13) a 19(15)-18(14) ~ 19(16)-18(15) a 20(14)-19(13) a 20(15)-19(14) a 21(13)-20(12) ~ 21(14)-20(13) ~ 21(15)-20(14) a 22(13)-21(12) a 22(14)-21(13) ~ 23(12)-22(11) ~ 23(13)-22(12) a 24(12)-23(11) ~ 24(13)-23(12) ~ 25(11)-24(10) a 25(12)-24(11) a 26(ll)-25(10) ~ 26(12)-25(11) ~ 27(10)-26(9) ~ 27(11)-26(10) ~ 28(10)-27(9) a 28(11)-27(10) a 29(9)-28(8) a 29(10)-28(9) a 30(8)-29(7) a 30(9)-29(8) ~ 30(10)-29(9) ~ 45(x, 45)-44(x, 44) b 45(1, 44)-44(1, 43) 45(2, 44)-44(2, 43) 45(5, 40)-44(5, 39) 45(6, 40)-44(6,39) 45(6, 39)-44(6, 38) 45(7)-44(7) 45(8)-44(8) ~ 45(9)-44(9) ~ 45(10)-44(10) ~ 45(11)-44(11) ~ 45(12)-44(12) ~ 45(13)-44(13) ~ 45(14)-44(14) c 45(15)-44(15) ~ 45(16)-44(16) ~ 45(17, 29)-44(17, 28) 45(17, 28)-44(17, 27) 45(18, 28)-44(18, 27) 45(19, 27)-44(19, 26)

G.S. 35C1

u = 1 350

61764.06 60534.61 63119.59 65704.59 61890.24 64475.18

61788.95 60557.20 63144.35 65731.03 61912.43 64499.33

60660.68 63245.69 65830.67 62016.07 64601.15 60786.36 63371.63 65956.59 62142.00 64727.00 60912.02 63497.21 62267.17 64852.34 61036.60 63622.33 62391.64 64977.71 61159.83 63746.36 62514.14 65101.07 61279.54 63868.29 60039.93 62632.85 65222.07 60271.95 60598.38 60585.86

60680.77 63267.63 65854.56 62035.63 64622.66 60803.68 63390.75 65977.80

61094.29 61103.29 60318.50 61051.43 61039.29 61030.74 61024.40 61019.45 61015.55 61012.59 61009.80 61007.59

G.S. 37C1 60870.27 62209.67 64753.58 63549.09 66092.95 62344.26 64888.38 61139.87 63683.83 62479.17 61274.46 63818.44 60069.57 62613.72 61408.83 63952.74 60203.39 62748.08 61542.24 64086.80 60336.02 61674.61

61804.43 60285.15 60607.16 60592.96 61161.36 60335.85 60338.82 61031.92 61020.57

61002.21 60998.43 60995.36

60306.71 60298.86 60292.91 60288.62 60285.15 60283.10 60280.58 60278.80 60277.13 60275.76

60989.22 60274.45 60273.36

304

W. Caminati et al. / Chemical Physics Letters 243 (1995) 302-307

Table 1 (continued)

J'(K'a, K 'c) '-- J"(K~, K~) 46(x, 46)-45(x, 45) b 46(1, 45)-45(1, 44) 46(2, 45)--45(2, 44) 46(5, 41)--45(5, 40) 46(6, 41)--45(6, 40) 46(6, 40)--45(6, 39) 46(8)--45(8) c 46(9)--45(9) c 46(10)--45(10) ¢ 46(11)--45(11) c 46(12)-45(12) c 46(13)-45(13) c 46(14)-45(14) c 46(15)-45(15) ~ 46(16)--45(16) c 46(17)--45(17) c 46(18)--45(18) ~ 46(19)--45(19) c 47(x, 47)-46(x, 46) b 47(1, 46)--46(1, 45) 47(2, 46)--46(2, 45) 47(5, 43)--46(5, 42) 47(5, 42)--46(5, 41) 47(6, 42)--46(6, 41) 47(6, 41)--46(6, 40) 47(8)--46(8) c 47(9)--46(9) c 47(10)--46(10) c 47(11)--46(11) c 47(12)--46(12) c 47(13)--46(13) c 47(14)--46(14) c 47(15)--46(15) c 47(16)--46(16) c 47(17)--46(17) ~ 47(18)--46(18) c 47(19)--46(19) ~ 47(20)--46(20) c 47(21)--46(21) c 47(22)--46(22) c 47(23)--46(23) c 47(24)--46(24) ¢ 48(X, 48)--47(X, 47) b 48(6, 43)--47(6, 42) 48(6, 42)--47(6, 41) 48(8)--47(8) c 48(9)--47(9) c 48(10)--47(10) c 48(11)--47(11) ¢ 48(12)--47(12) c 48(13)--47(13) c 48(14)--47(14) c 48(15)--47(15) c 48(16)--47(16) c 48(17)--47(17) c

G.S. 35C1 61607.08 61932.46 61921.55 62568.53 62455.35 62466.76 62410.00 62396.86 62387.70 62381.16 62375.75 62371.63 62368.07 62365.48 62362.91 62361.16 62359.02 62357.82 62942.63

v = 1 35C1 61620.80 61940.95 61929.40 62533.00

62377.92

62956.42

G.S. 37C1 61318.63 61299.44

61647.78 61639.00 61633.13 61628.55 61625.02

62346.58 62639.50 62623.20

63808.55 63943.85 63816.38 63830.50 63768.62 63754.73 63744.85 63737.52 63732.07 63727.65 63724.02 63720.93 63718.64 63714.38 63712.82 63711.18 63709.64 63708.36 63707.23 63706.28 64277.59 65177.40 65195.25 65127.32 65112.51 65102.15 65094.29 65088.45 65083.82 65080.00 65076.75 65074.09 65071.82

63804.51 63747.76 63734.98 63725.89

62979.74 62973.13 62968.33 62964.63 62961.76 62959.36 62955.37 62953.76 62952.49 62951.22 62950.99

64292.23

63669.34

65106.06 65092.29

64330.31 64320.58 64313.53 64308.33 64304.36 64301.09 64298.04 64296.04 64294.51 64292.23

65061.07 65058.96 65056.89 65054.10

W. Caminati et al. / Chemical Physics Letters 243 (1995) 302-307

305

Table 1 (continued) J ' ( K 'a, K'c) ~ J"(K~, K~)

48(18)-47(18) 48(19)-47(19) 48(20)-47(20) 48(21)-47(21)

G.S. 35C1

c c c c

v = 1 35C1

65069.75 65068.13 65066.40 65065.20

G.S.

37C1

65052.49

Asymmetry degenerate K_ 1 /xc-type transition doublets. b Asymmetry degenerate K+t transition doublets; x = 0 or 1. c Asymmetry degenerate K_I /G-typ e transition doublets.

few Pa). To obtain a suitable concentration of the sample in the carrier gas it was necessary to warm it up. The heating device and the details of the spectrometer are given elsewhere [5]. In the experiment the sample seeded in argon at a stagnation pressure of = 20 kPa at 100°C was expanded to about 0.05 Pa through a 0.35 mm diameter nozzle. Trial values of the rotational constants have been calculated from the structural parameters of Ref. [1] for the two limiting cases z = 0 or 90 °. In the former case /.G-and /.tb-type transitions would have been expected, while they would have been /x a and /x c type in the latter case. jiza R-type band transitions were easily observed for both 35C1 and 37C1 isotopomers when applying a Stark field of 2 0 0 - 4 0 0 V / c m . After the statistical error on the rotational constant A was of the order of 1 MHz it was possible to assign several /x c R-type transitions. The same kind of transitions were measured also for a vibrational satellite of the normal

species. Their intensities were about one third of those of the ground state. The experimental frequencies are listed in Table 1. They have been fitted with a quartic Watson Hamiltonian [6] giving the spectroscopic constants in Table 2, together with some statistical parameters of the fits. From the rotational constants of the ground states of the two isotopic species the r s located position [7] of the C1 atom has been obtained. The corresponding coordinates are given in Table 3. The zero value of the b coordinate shows unambiguously that in the ground state the C I - S bond is in a plane orthogonal to the benzene ring plane. In Table 3 a partial r 0 structure is also given. The geometry from electron diffraction [1] has been used as a starting point and the parameters of the SO2C1 group have been refined in order to reproduce the rotational constants. As to the observed vibrational satellite, we think that it should belong to the SO2C1 group internal

Table 2 Rotational and centrifugal distortion constants of BSC G.S. 35C1 A (MHz) B (MHz) C (MHz) Aj (kHz) AjK (kHz) a,v (kHz) 6j (kHz) 6K (kHz) Hj,~ (Hz) H x j (Hz) N b o- (MHz)

1970.63(1) a 687.79(1) 668.05(1) 0.027 (1) 0.24(2) - 0.08(2) 0.001 (1) -5.3(2) 0.031 (4) -0.11(1) 104 0.11

a Errors in parentheses are expressed in units of the last digit. b Number of transitions in the fit.

v = 1 35C1 1971.39(1) 687.27(2) 668.19(1) 0.025(2) 0.155 (6) - 0.06(3) 0.005 (2) - 4.9(2)

33 0.15

G.S. 37C1 1942.00(1) 677.82(2) 661.84(2) 0.023(2) 0.118(4) 0.005(2) 0.005(2) -5.3(3)

69 0.20

W. Caminati et al. / Chemical Physics Letters 243 (1995) 302-307

306

Table 3 Structural parameters (,~ and deg) of BSC Partial r o structure a Bonds rc_c rc_n rs_ o rc_s rs_o

b c d d

Angles 1.403 1.084 1.417 1.737 2.023

101.4 107.7 90 114.5

CSC1 CSO d CIS-C1C / (z) OSC-CSCI d

C1 r s coordinates Exp.

Calc. e

lal

1.883

1.894

Ibl

0

0

[cl

1.392

1.390

a The discrepancy between the experimental rotational constants and those calculated with this geometry are of the order of 1 MHz. b From Ref. [1]; regular structure of the benzene ring. c Assumed. d Parameters adjusted to reproduce the experimental rotational constants. e Calculated with the above geometry.

rotation. We have never observed vibrational satellites, except in cases concerning excited states due to a two equivalent minima doubling [8]. For example in the case of syn allyl alcohol [5,8] the vibrational splitting between the O + and O - sublevels is about 0.5 cm-1 and we were able to estimate the 'vibrational' temperature, which was about 5 K. It is comparable to the usually observed 'rotational' cooling ( 7 - 8 K, Ref. [5]). Such a vibrational cooling for our vibrational satellite would correspond to a vibrational energy of 6 cm -1. We tried to use this value and the shifts of the second moments of inertia with

Table 4 Flexible model results for BSC Shift of second moments of inertia (uA 2) Exp.

Calc.

AMaa A Mbb AMcc E~= 1

0.249 -- 0.407 0.307 -

0.202 -- 0.532 0.388 31.8

Obtained parameters Vz = 900 c m - 1

8 = 4.4 °

respect to the ground state upon torsional excitation to obtain information on the potential energy function of the SO2C1 group internal rotation by using a flexible model [9]. This scheme allows the calculation of the rotational constants, the energies and the wavefunctions for the overall momentum quantum numbers J = 0, 1 in the vibrational excited states. The second moments of inertia, related to the rotational constants through M a a = / 8 a ' r ( 1/ A + 1 / B + 1/C), etc., are useful in visualizing the mass extension along the principal axes. Owing to the scarcity of data, we used the simplest potential energy functiod suitable for our problem, V ( x ) = 1V 2 [1 - c o s ( 2 x ) ] .

(1)

Here x is 0 for the equilibrium configuration (that is x = r + 90). After a few calculations we realized that the vibrational energy of 6 c m - 1 was not compatible with the shifts of second moments of inertia, so that, just from these latter data, we estimated the V2 parameter together with the structural relaxation of the CSC1 angle ( a ) associated to x, according to ot(deg) = 101.4 + ½611 - c o s ( 2 x ) ] ,

(2)

where 101.4 is the r 0 value (deg) and 6 represents the increase of the angle when x = 90 ° (~-= 0). The three shifts AMgg ( g = a,b,c) of second moments of inertia in going from the ground to the excited state, have been fitted to determine the two model parameters. The results of the application of the flexible model are shown in Table 4. The vibrational energy of the first torsionally excited state is also given. If our model is correct the vibrational cooling would than have been to about 42 K. In the numerical calculations the range rr was resolved in 33 mesh points [9]. We cannot rule out the possibility that the observed vibrational satellite belongs to another mode for instance a vibrational state with energy greater than kT - which has not been 'cooled' during the adiabatic expansion, but we did not observe such effects in our previous investigations. As a conclusion we have to remark that Boggia and co-workers [2,3], as outlined in Ref. [4], were very optimistic in assigning the conformation on the basis of the L R M W spectrum. Fig. 2 of Ref. [2] -

W. Caminati et al. / Chemical Physics Letters 243 (1995) 302-307

reports this spectrum. A broad band was assigned to some accidental piling up /xb-type transitions. Actually it is a /xc-Q-branch band with K_ 1 = 4 ~ 3. This kind of band is spaced 2 A - B - C [10]. The electron diffraction value ( r = 75 °, Ref. [1]) reflects the contribution of the excited states, important when a large amplitude motion takes place, to the average structure. For ethylbenzene the same kind of deviation between the MW (r = 90 °, Ref. [11]) and the ED ( r = 70 °, Ref. [12]) investigations was observed. In Ref. [4] arguments are given to show that the L R M W spectrum better fit the r = 90 ° rather than the ~'= 0 ° conformation. They are based on the model differences (A) of the (B + C) ° parameters between the 35C1 and 37C1 species at various values of z. Probably because of a typographical error all the A have the same value [4], but apart from this fact we think that the argument is not so convincing because the considerable structural relaxations of the SO2C1 group associated with the value of r were not taken into account. This work shows the utility of the free jet technique in studying conformational equilibria of molecules which have congested spectra due to high molecular weight and low energy-large amplitude motions.

307

The Ministero dell'Universith e della Ricerca Scientifica e Tecnologica, and the Consiglio Nazionale delle Ricerche are acknowledged for financial support.

References [1] J. Brunvoll and I. Hargittai, J. Mol. Struct. 30 (1976) 361. [2] L.M. Boggia, R.R. Filgueira, J. Marafion and O.M. Sorarrain, Spectry. Letters 11 (1978) 143. [3] C.H. Gomez, R.R. Filgueira, L.M. Boggia and O.M. Sorarrain, An. Soc. Cient. Argent. 205 (1978) 17. [4] B.P. Van Eijck, I. Hargittai and I. Mayer, J. Mol. Struct. 69 (1980) 301. [5] S. Melandri, W. Caminati, L.B. Favero, A. Millemaggi and P.G. Favero, J. Mol. Struct. 352/353 (1995) 253. [6] J.K.G. Watson, in: Vibrational spectra and structure, Vol. 6, ed. J.R. Durig (Elsevier, Amsterdam, 1977) pp. 1-89. [7] J. Kraitchman, Am. J. Phys. 21 (1953) 17. [8] S. Melandri, P.G. Favero and W. Caminati, Chem. Phys. Letters 223 (1994) 541. [9] R. Meyer, J. Mol. Spectry. 76 (1979) 266. [10] W. Caminati and D. Damiani, J. Mol. Struct. 147 (1986) 389. [11] W. Caminati, D. Damiani, G. Corbelli, B. Velino and C.W. Bock, Mol. Phys. 74 (1991) 885. [12] P. Sharfenberg, B. Rozsondai and I. Hargittai, Z. Naturforsch. 35a (1980) 431.