FTIR spectrum of benzene in a supersonic expansion

CHEMICAL PHYSICS LETTERS

Volume 103, number 5

FTIR SPECTRUM

OF BENZENE

Deanne L. SNAVELY,

IN A SUPERSONIC

Valerie A. WALTERS,

13 January 1984

EXPANSION

Steven D. COLSON and Kenneth

B_ WIBERG

Sterling Chemistry Lubomtory, Yale University. New Harden.Connecticut 06511. USA

Received 25 July 1983; in final form 26 October

The 700 and 3050 troscopy_ Analysis of Trot (79 f 15 K) and dom. A band contour

1983

infrared absorption speccm-l region of benzene seeded m argon is studied using Fourier-transform the sequence band structure in the 700 cm-l region gave new anharmonic constants and values of Tyib (==I60 K) which indicate a disequilibrium between the rotational and vibrational degrees of free-

analysis of the 3050 cm -* Fermi triad is used to obtam new values of the Coriolis coupling constants

and band origins

1. Introduction

quencies can be determined. These improvements. demonstrated by this work on benzene, should facilitate the vibrational analysis of other molecules by distinguishing between fundamentals, combinations and hot bands. Along with these advantages realized in the analysis of the infrared absorption spectrum, we can also study the rotational and vibrational cooling processes_ The shape of the rotational band contour determines the Trot. rvib is obtained from the sequence hot band intensity. in this paper_ we report our studies of the rotationvibration spectmm of benzene cooled in a supersonic expansion. We have obtained the infrared absorption spectrum of the 700 cm-l (A?,) band, which in Wilson’s numbering system is v1 1, and have studied the vibrational cooling evident in this regjon. Also. a spectrum of the C-H stretching region, at 3050 cm-l. was obtained in which the Fermi triad of overlapping bands was separated.

Rotational and vibrational cooling, afforded by the expansion of polyatomic molecules in a supersonic jet, have been used in many instances to simplify the electronic absorption and emission spectra. This simplification can also be advantageous in the study of rotationvibration spectra. A method for obtaining the broadband infrared absorption spectrum of polyatomic molecules expanded in a supersonic jet has been developed using Fourier-transform infrared (FTIR) spectroscopy_ The Nicolet 7 199 FTIR, a sensitive, accurate (0.01 cm-l), high-resolution (0.06 cm-l) spectrometer, was combined with a supersonic jet machine. With this technique a wide spectral range (400-4000 cm -l) can be studied and, by signal averaging, absorbances as low as 10e3 can be obtained. Both neat ammonia and methyl chloride have been studied with this tech-

nique [ 1,2] _In the first case, we studied the rotational cooling in the v2 band and, in the second, we also determined the rotational constants using a band contour analysis. The two important improvements in the free jet infrared spectrum are the separation of overlapping bands due to rotational cooling and the decrease in the intensity of sequence hot bands due to vibrational

2. Experimental The experimental apparatus has been previously described [ 1 ,z] so that only a brief description will be

given here. Asupersonic jet of benzene seeded in argon was prepared by passing the gas mixture through a nozzle orifice into a vacuum chamber which was maintained below 1 X 1O-3 Torr. For the 700 cm-l region

cooling. These improvements aid in the determination of the band contour of the C-H stretching absorptions

from which rotational

constants

and vibrational

fre-

0 009-26 14/84/S 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

423

Volume103,number5

CHEMICAL PHYSICSLETTERS

experimental spectrum, 0.5 atm of argon was bubbled through a heated vessel containing liquid benzene where the vapor pressure of the benzene was 1 atm. The gas mixture was then expanded through a nozzle with a 1 mm diameter hole. For the 3050 cm-r region, 1 atom of argon was mixed with 0.75 atm benzene and passed through a 0.5 mm nozzle orifice. The nozzle inlet line was kept about 20°C above the heated vessel temperature to inhibit condensation. The infrared light from the Nicolet 7 199 spectrometer passed at right angles through the supersonic jet al-4 mm downstream from the nozzle. A HgCdTe detector recorded the 700 cm-l region while an InSb detector, with its excellent response in the near infrared, was used for the C-H stretch region. In the C-H stretch region, optical filters, in addition to the electronic filters provided by the FTIR, were used, in order to increase the S/N. A coated quartz, long bandpass filter passed only light in the 4300-2600 cm-l interval_ To obtain the spectrum of the 700 cm-l region, 140 signal scans and 770 background scans were signal averaged, at a resolution of 0.06 cm-l. Due to the low S/N in the C-H stretch region, the spectrum was recorded at 0.5 cm-l resolution with 1150 signal scans averaged together and ratioed to 1700 background scans. Intense infrared light was provjded by a heated car$ond rod source based on the design of Baa [3]. A specially machined pure carbon rod was resistively heated by passing 200-250 A at 4-5 V across the rod. This rod was mounted on two thick copper electrodes and housed in a chamber which was pressurized to 2 atm with argon in order to increase the lifetime of the rod. The housing and the electrodes were water cooled. The rod operated from 2400 to 2600 degrees and lasted about 100 h. With this source, the total infrared signal more than doubled at ~3000 cm-l so that electronic and optical filters could be used to limit the bandpass of light which reached the detector, while still maintaining a strong total infrared signal. In this way, the S/N was increased. In order to discourage condensation in the free jet, we uselow stagnation pressures, a large orifice diameter. and a heated nozzle_ While it is likely that benzene-benzene and benzene-Largon clusters were formed in our beam, none were detected. Argon has been found to encourage cluster formation in supersonic jets where argon was added to the molecule in 424

13 January 1984

helium [4] _The previous studies of benzene clusters in supersonic jets [5,6], in addition to using different beam conditions than in this work, used sensitive detection techniques which given information mainly about the clusters.

3. Band contours The analyses of all these spectra were done using computer programs which simulate the rotational band contours. The symmetric top program had been used before [ 1,2] but was improved for this study to include Coriolis coupling terms [7] _ The symmetric top line positions and intensities were then convoluted with a gaussian function having a fwhm to simulate the resolution of the experimental spectrum. The overall shape of some types of rotational band contours has been observed to change drastically with a change in the temperature. Clouthier and Birss [8] have shown that the band maximum of the 46 band of the A lA1 -X IAl transition in thiophosgene shifts by l-2 cm-l and that the R and P branches changed in relative intensity to each other in going from 300 to 25 K. This problem can lead to a faulty interpretation of the beam spectra [9] and will be particularly troublesome in regions of overlapping bands where several contours have changed simultaneously. For this reason, we checked the constants which were obtained using the low-temperature spectrum by using them to simulate the room-temperature spectrum where the observed bands overlap.

4. Previous work Benzene has four infrared active fundamentals, with only one AZ,, mode and three E,, modes. The infrared spectrum of benzene has been studied many times [lo- 171 and the band contours for these four fundamentals have been simulated [ 131 at a resolution of 1 S cm-r. Danti and Lord [ 1 l] were able to distinguis and assign some of the sequence bands of vll in their 0.3 cm-r resolution spectrum. However, they were unable to identify the 13C12CgHg peak. Highresolution work on yll was done at room temperature and at -50°C at 0.03 cm-l resolution by Cabana et al. [15] _ These workers were able to distinguish the hot bands using their increased sensitivity and low

13 January

CHE&lICAL PHYSICS LE’ITERS

Volume 103, number 5

1984

temperature. Daunt and Shurvell [ 131 studied vll and followed the same sequence band assignments as Danti and Lord. Most recently, Kauppinen et al. [ 161 in their determination of B. for benzene from vtl band analyzed the rotational structure of the first member of the v16 sequence band and reported X1 1 16. Daunt and Shurvell also simulated the rotational bland contour of the v2,-, fundamental in the 3050 cm-l region but did not attempt to simulate the combination bands which are strongly mixed with the fundamental. These workers also assigned the sequence bands of their spectrum which was taken at 0.2 cm-l resolution. In addition, the C-H stretch fundamental of benzene and its overtones have been studied by Berry et al. [ 181. In their

analysis,

a bond

was used. The Doppler-limited “20 from 3020 to 3 125 cm-l

contour

Pliva and Prne [ 171 at a temperature further

deconvolution

calculation

absorption spectrum of has been analysed by

of the spectrum

of 204 K. By a resolution

of

0.0010-0.0015 cm -l was attained so that the rotational structure could be fully resolved and, through the analysis of 4300 lines, the upper and lower state constants were obtained_ The supersonic expansion of benzene has been studied previously using electronic spectroscopy_ Workers [ 191 have found substantial cooling using a PD = 9 atm mm with 0.1% benzene in helium and determined the vibrational frequencies and rotational constants for the 1 Bzu- lAlu transition. The multiphoton ionization spectrum of a free jet expansion of benzene seeded in helium was obtained by Johnson [20] where he studied the 14615 progression of the excited state originating from the vibrationally cold ground state. The narrow rotational linewidth allowed comparison of the natural linewidth of various transitions to learn about radiationless transitions_ LangridgeSmith et al. [21] studied benzene clusters noting that in their fluorescence excitation spectrum vibrational cooling was incomplete. They observed transitions from u = 1 in v(j and found no vibrational cooling had occurred. Also, Tvib based on the VI6 hot band intensity was about 200 K.

5. This WOlJr 5.1. The 700 cnl-I The experimental

region, vlI

room-temperature

I 696

684

660

672

Wavenumbers

(cm-‘)

Fig. 1. Room-temperature spectra of the 700 cm-1 region in benzene. (a) Simulated spectrum: 0.1 cm-l resolution, rotationaI constants from ref. [15]. (b) E\perimtntal spectrum.

7 Torr benzene, 0.06 cm-’ resolution.

along with its simulation, is shown in f& 1- The e-xperimental spectrum was simulated with the symmetric top program using Giguere‘s .N3 with the predicted intensities at room temperature for the various sequence hot bands. The expected relative intensities based on the Boltzmann factors are 1 : 0.292 : 0.108 : 0.039 : 0.042 for the ull : vIl + v16 - v16 : vll + v6 - v6 : 2vll - VI1 : VI1 + 2v16 - 2v16 bands, respectively. The nuclear spin statistics are rhose given by Wilson [22]. The frequencies and assignments of each of these five bands are given in table 1. There are two members of the 402 cm-l sequence band progression observable at room temperarure. The second member overlaps another peak. We have assigned this peak to the 2ull - vll band because it has the correct intensity and is expected to be the next most intense band according to the Boltzmann factors given above. Daunt and Shurvell assign the 21, 11 - vll band to the side band on their main Q branch at 675.3 cm -l which does not appear in our

spectrum, 425

13 January 1984

CHEMICAL PHYSICS LETTERS

Volume 103. number 5

Table 1 Frequencies and assignments for the 700 cm-’ region of benzene Assignment

Room-temperature spectrum

frequency VI1 =C Vll +X+j -n,5 Vll +"16

-u16

Yll +3,16

-

2 VI1

2Y16

-v11

_

Free jet spectrum

relative intensity

frequency

relative intensity

673.99 673.61

1 0.071

673.98 673.60

1 0.052

673.48

0.108

-

-

672.87

0.292

672.88

0.052

671.82 (671.85) 671.73 (671.70)

0.042

-

0.039

671.70

Xi.11

0.007

Sequence band shifts and anharmonic constants Sequence band

Vi’ 1

Vi= 2

“16 v6

-1.11 -0.5 1 -2.29

-2.14

JJ11

low-temperature spectrum, shown in fig. 2. It is impossible to determine which half of the double peak is

the vll + 2~~~ - 2v16 or the 2~~~ - vll from the room-temperature spectrum. However, in the jet spectrum only the lower energy peak remains. This is probably the 2~~~ - yll sequence band since this absorption originates from a state which lies 131 cm-l below the 2~16 state (2~~~ - z;ll = 804 - 673 cm-l). The frequency of 671.70 cm-l, as determined from the jet spectrum, wasused as the origin for the vll hot band and the value of 671.85 cm-l was used for the v16 sequence band. When these two origins (shown in parentheses in table 1) were used for these overlapped bands, the simulation gave a good fit. One of the observed peaks was identified as the isotope peak for 13C12C5H6 because it had the expected 6.6% of the total intensity, and did not decrease in relative intensity in the jet spectrum. The isotope shift for 13C12C5H6 was calculated by Laufer and Kopelman [23] for use in their crystalline benzene studies, to be 0.33 cm-l _Our experimental determination of 0.37 cm-l, from the jet spectrum, agreed well with this value. 426

-1.11 -0.51 -2.29

The Q-branch intensity of v1 1 was ignored in this simulation because it is undoubtably incorrect due to saturation effects resulting from our limited resolution. In this simulation, we fit the intensities of the P and R branches whose intensities relative to the Qbranch intensities of the sequence bands, should be accurate in that they have comparable peak widths and heights. Accordingly, the simulated Q branch for vll in fig. 2 was much higher than the experimentally observed intensity. For the supersonic jet spectrum, the rotational temperature was determined for yll by fitting the Rand P-branch intensities using a least squares program described earlier [2] _ The Trot was found to be 79 + 15 K. This is cooler than the temperatures found for the-neat ammonia and methyl chloride from the previous work. In the jet spectrum, only the first members of the band progressions were left in addition to that of the isotope. The relative intensities of these remaining bands, were determined by simulation. The experimental and simulated spectra are shown in fig_ 2 and the intensity ratios are in table 1. The vibrational tempera-

CHEMICAL

Volume 103, number 5

PHYSICS LETTERS

13 January

1984

The anharmonic constants for the vll combination bands can be determined from the sequence hot band positions. These are also given in table l_ The value of xl6 11 (-1.11 cm-l) agrees with that reported by Kap’pinen et al. [ 161.

a

5.2. l7re 3050 c~z-~ region, vz0

b

617

661

675

Wavenumbfxs

669

663

657

km”)

Fig. 2. Jet spectra of the 700 cm-’ region in benzene. (a) Simulated spectrum, 0.1 cm-l resolutcon, Trot = 95 K, vg = 673.98 cm-’ . (b) Experimental spectrum, 0.06 cm-’ resolution, 1 atm benzene in l/2 atm argon, 1 mm nozzle orifice.

ture using the observed

member of the “16 sequence was found to be 158 + 20 K. The 671.8 cm-1 peak, which is a double peak at room temperature, is now definitely only one peak, shifted 0.03 cm-l to lower frequency. The relative intensity of this band, which we have assigned to the 2vll - vll transition, gives Tvib = 171 -C30 K. The sequence band for the 607 cm-1 band, vs, is completely gone. If we assume that the largest this peak could be is as large as the noise, then T,ib(YS) < 145 K. Even given the large error in the estimation of Tvib, we see that the TYib > Trot for benzene, and that Tyib(“ll) z Tyib(v16) 2 Tvib(v6)_ It is interesting to note that Levy [9] observed very little cooling of the v6 mode in a supersonic jet of dihrte benzene in a helium carrier gas and determined Tvib(v6) to be x200 K. We definitely observe v6 vibrational cooling, along with cooling in v16 and vll-

The C-H stretch region of benzene has three overlapping absorptions at room temperature. The strongest at 3047.3 cm-l is assigned to the fundamental [ 131, vzo_ The other two bands are combination modes which derive their intensity through a strong Fermi resonance with the fundamental mode. These two other modes are v8 + vIg reported to be at 3082.4 cm-l and VI + “6 + “19 at 3101.8 cm-l_ All three of transitions. these bands are E lu, perpendicular Our analysis began with the supersonic jet spectrum shown in fig. 3 which shows that the fundamental was no longer overlapped by the combination bands and the middle band was well separated so that the band contour is discernable. Based on our knowledge of the cooling from the 700 cm-l region, we also assert that the Trot = 79 I 15 K. The band contour of the room-temperature spectrum of the vZo fundamental has been simulated before by other workers [ 13.17,1 S] _Using the rotational constants and Coriolis constant obtained by the detailed analysis of uZo by Pliva and Pine, a good fit was obtained when simulating our jet spectrum of this band. The values of A3 and AC used in the simulation of all three bands were the same (see table 2). By varying 520 by 50.05 cm-l no appreciable

change in the simulated spectrum could be observed at our experimental resolution. We fit the Coriolis constants for v1 + v6 + v19 and vS + v19 to our cold spectrum. The constant for “8 + VI9 was quite large and opposite in sign. whereas that of the v1 f us + vrg band is similar to fZo_ The calculated contours where T rot = 80 K and a resolution of 0.5 cm-l was used, are shown in fig. 3. The relative intensities of these three bandsare 1 : 0.3 : 0.8 for vZo, v8 + vlg and vl + “6 + v19_ The previously reported [ 131 origins for the two combination bands at 3082.4 and 3102.4 cm-l do not fit our low-temperature spectrum_ which show the origins to be at 3078.56 cm-l and 3 100.75 cm-l, respectively. Given the rotational constants determined from 427

CHEMICAL PHYSICS tilI’ERS

Volume 103, number 5

13 January 1984

ture is apparent in the calculated spectrum, however, where sequence bands were neglected.

6. Conclusions From the study of benzene in the supersonic jet, we have obtained new Coriolis constants and vibrational term values. This was possible because of the rotational cooling which separated the three overlapping bands in the C-H stretching region. Since these bands were separated, the rotational band contours could be simulated and compared to the experimental spectrum.

These contours

were fit quite well with

only three constants, LU3, AC and 5, in the symmetric top formalism, where the error in the 5 was 0.05 cm-l. It is important to keep in mind that we have not

Fig. 3. Spectra of the 3050 cm-l region in benzene. (a) Experimental jet spectrum, 0.5 cm-’ resolution, 314 atm benzene in 1 atm argon, InSb detector, carbon rod source. (b) Simulated jet spectrum, 05 cm-1 reblution, Trot = 80 K, the constants and relative intensities used for these bands are sbown~intable 2. (c) Experimentat room-temperature spectrum. 7 Tqn. 0.06 cm-l resolution. (d) Simulated roomYternperature spectrum, 0.1 cm-t resolution, Trot = 300 K, relative intensitiesgiven in table 1.

the jet spectrum, the room-temperature high-resolution (0.06 cm-l) spectrum can be simulated. For this simulation the same origins and relative intensities were used as in the simulation of the jet spectrum. The differences between the observed and calculated spectra are quite small (fig. 3), which shows that any band shape distortions due to hot bands in the room-

temperature spectrum are minimal. More fine struc-

Table 2 Origins, constants and relative intensities for the 3050 cm-l Assignment -.

Frequency

v20

3Wj.62

3078.56 3100.75

.

va + 49

v;+v6+v1g a) Determined in this work.

428

resolved the individual lines, Pliva and Pine have shown that the Doppler-l&ted width of the lines in ~2~ at 204 K is = 3.6 X 1 Om3 cm-l, while the resolution of our cold spectrum is 0.5 cm-‘. The overall band width of ~2~ in our jet spectrum is ~18 cm-1 as compared to -35 cm-1 at room temperature. The two combination bands sharpen up in a similar way. l-he =ro t for benzene was colder than the Trot of methyl chloride and ammonia in our earlier work. This difference was due to the 1 : 1 ratio of argon to benzene which increases the heat capacity ratio of the gas and results in more cooling. Vibrational cooling in benzene was also observed through the simplification of the sequence band structure in the 700 cm-l region of the experimental spectrum. previous workers observed little vibrational cooling when helium was used as the seed gas. Our work confirms the expecta-

tion that benzene-benzene and argon-benzene collisions are more effective than benzepe-helium collisions in cooling the internal degrees of freedom, be-

region for benzene (y-cc’

5

0.00021

0.00019

-0.0984

1.0

0.00021 0.00021

0.00019 0.00019

0.70 2 0.5 a) -0.10 * 0.5 a)

0.3 0.8

B”-B’

Relative intensity

CHEhIICAL

Volume 103. number 5 cause benzene the helium.

13 January

PHYSICS LETTERS

and argon are both more polarizable

Acknowledgement The authors gratefully acknowledge financial support of the National Science Foundation CHESOl7304 and a Graduate Fellowship from The Hey1 Foundation for V.A.W.

1984

[ 81 D.J. Clouthier and F.W. Buss, Chem. Phys. Letters 86 (1982) 389. [ 9) R. Vasudev, Y. Hirata. EC. Lim and W.M. M&lain Chem. Phys. Letters 76 (1980) 249. [IO] S. Brodersen and A. Lan8seth. K;gl. Da&e Videnskab. Selskab. Mat. Fys. hledd. 1 (1956). [ 111 A. Danti and R.C. Lord, Spectrocbim. Acta 13 (1958) 180. [12] D. Steele and W. Wheatley, J. Mol. Spectry. 32 (1969) 260. [ 131 S.J. Daunt and H.F. Shurvell, Spectrochim. Acta 32A (1975) 1545.

[ 141 CR. Chary. V.B. Addepalli. V.A. Padma and N.R. Rao. References [1] D.L. Snavely, S.D. Colson and K-B. Wiberg. J. Chem. Phys. 74 (1981) 6975. [2] D.L. Snavely, K.B. Wiberg and S.D. Colson. Chem. Phys.

Letters 96 (1983) 319. [3] G.R. Smith, B. Fridovitch,

Indian J. pure Appl. Phys. 16 (1978) 526. [151 A. Cabana, J. Bachand and J. Giguere. Can. J. Phks. 52 (1974) 1949. 1161 J. Kauppinen, P. Jensen and S. Brodrrsen, J. Mol. Spectry. 83 (1980) 161. t171 J. Pliva and AS. Pine, J. Mol. Spectry_ 93 (1982) 209. [=I R.G. Bray and M.J. Berry, J. Chem. Phys. 71 (1979) 4c90;

W. lvancic and R.N. Rae,

Rev. Sci. Instrum. 49 (1978) 1223. [4] M.P. Casasa, D.S. Bomse and K.C. Janda, J. Chem. Phys.

74 (1981) 5044. [5] J.B. Hopkins, D.E. Powers and R.E. Smalley, J. Phys. Chem. 85 (1981) 3739. [6] M.F. Vernon, J.M. Lisy, H.S. Kwok, D.J. Krajnovich, A. Tramer, Y.R. Shen and Y.T. Lee, J. Phys. Chem. 85 (1981) 3327. [7] G. Her&erg, Molecular spectra and molecular structure, Vol. 2. Infrared and Raman spectra of polyatomic

molecules (Van Nostrand, Princeton,

I191

K.V. Reddy, D.F. Heller and MJ. Berry, J. Chem. Phks. 76 (1982) 2814. S.hl. Beck, h1.G. Liverman, D.L. Mont and RX. Smalley. J. Chem. Phys. 70 (1979) 232.

Phys. 19 (1980) 3920. [=I P.M. Johnson,AppL 1211 P.P.R. Langridge-Smith, D.V. Brumbaugh, CA. Hayman and D.H. Levy, J. Phys Chem. 85 (1981) 3742.

[22] E.B. Wrlson, J. Chem. Phys. 3 (1935) 276. P31 J-C. Lauffer and R. Kopelman. J. Chem. Phys. 57 (1972)

3202.

1945) p_ 401.

429