- Email: [email protected]
High-resolution FTIR and diode laser jet spectroscopy of cyclopentadiene
JOURNAL
OF MOLECULAR
SPECTROSCOPY
143, IOO- 110 ( 1990)
High-Resolution FTIR and Diode Laser Jet Spectroscopy of Cyclopentadiene STEVEN R. BOARDMAN,
STEWART A. BONE, PAUL B. DAVIES,
AND NICHOLAS
A. MARTIN
Department of Chemistry, University of Cambridge, LensJield Road, Cambridge CB2 1E W, United Kingdom High-resolution (Doppler-limited) infrared absorption spectra of the 26 h band of cyclopentadiene ( C5H6) have been recorded with FfIR and diode lasers. In addition to room temperature measurements, diode laser spectra were also recorded in a molecular jet. The latter experiments were particularly useful in revealing the low-N transitions in the Q branch. Analysis of 420 P- and Rbranch lines of this C-type band, with the ground-state constants fixed at microwave-determined values, yielded rotation and distortion parameters for the upper (26,) level. The derived band origin, Z0 = 663.84800( 5) cm-r, is in good agreement with that from the vibrational analysis at lower KSOhtiOn. 0 1990 Academic Press. Inc. INTRODUCTION
Cyclopentadiene (CPD) and its derivatives have played an important part in organometallic and structural chemistry. It is the prototype molecule for many other five-membered rings containing two double bonds, including heterocycles, which have been widely studied by electron diffraction and microwave spectroscopy. In the latter case the derived spectroscopic parameters have been linked to the potential energy surface and hence the conformation of the molecule ( 1). Cyclopentadiene itself has been extensively studied by microwave spectroscopy (2-5) culminating in the complete determination of its r, and r. structures by Damiani and co-workers (4). They used several deuterated CPD isomers and the earlier structural data of Scharpen and Laurie (3) to achieve this. More recently, measurements were extended into the millimeter region for astrophysical purposes (5) following the discovery of other small ring molecules in the interstellar medium. In contrast to the numerous microwave studies, considerably less has been reported on the infrared spectroscopy of CPD. In the gas phase, IR studies have been restricted to a vibrational band analysis, which in conjunction with Raman spectroscopy led to the assignment of the 27 fundamentals (6). The IR spectra of the vapor recorded by Gallinella et al. (6) showed considerable band overlap above ca. 700 cm-‘. However, at lower wavenumbers, two fundamentals, the ring bending mode at 350 cm-’ (27 A, B2)and the CH out-of-plane bending mode of the vinyl hydrogen atoms at 664 cm-’ (26;, B2), w ere well resolved. This article reports a study of the latter fundamental using FTIR at room temperature, with complementary diode laser jet spectroscopy to simplify regions where poor signal-to-noise ratios or spectral congestion made the FIIR spectrum less useful. The jet spectra also provided a rigorous test of the derived spectroscopic parameters which were used to simulate representative regions of the 0022-2852190 $3.00 Copyright 0
1990 by Academic Press, Inc.
All rights of reprcduction in any form reserved.
100
HIGH-RESOLUTION
101
IR OF CYCLOPENTADIENE
diode spectra. In addition to providing the first rotationally resolved IR spectrum of CPD, this study is of general relevance for extending IR spectroscopy to near-oblate tops for which very few high-resolution studies have been reported ( 7). EXPERIMENTAL
DETAILS
Cyclopentadiene was prepared by the thermolysis of the dimer (dicyclopentadiene, Aldrich, 95% purity) over copper (8) and its purity checked by low-resolution FUR. On the time scale of the experiments reported here ( 13 h in the case of some FTIR spectral recordings) no significant dimerization occurred. FTIR spectra were recorded at an unapodized resolution of approximately 0.003 cm-’ on a BOMEM DA3.002
(a)
(d)
cc: 663.7
663.8
663.9
cm-’
663.7
663.8
,‘.
663.9
.
.
.
(
cm-’
FIG. 1. Diode laser spectra of the Q-branch region of the 26; mode of cyclopentadiene. (a) Room temperature spectrum in a 1km cell at 2 mm Hg pressure. (b) Jet-cooled spectrum using 5% CPD in Ar. (c, d) Corresponding simulations using the parameters in Table II at temperatures 7’,, = 298 and 25 K. respectively.
102
BOARDMAN
ET AL.
spectrometer using a Si:B detector and a KBr beam splitter, along with a 15-cm-long single-pass cell filled to a pressure of0.2-2 Torr. Room temperature diode iaser spectra were recorded under similar conditions. The experimental arrangement for diode laser jet spectroscopy has been described elsewhere (9). Pure CPD or mixtures with He or Ar at stagnation pressures of up to 2 atm were expanded into the vacuum chamber via a Newport Corporation pulsed valve ( 192 Hz, l-mm-diameter orifice). The diode laser beam traversed the jet up to
(d)
h 660.47
660.49
660.51
FIG. 2. Diode laser spectra Corresponding simulations.
660.53
of jet-cooled
cm”
657.62
pure CPD: (a) P(6)
657.67 transition,
657.72 (b) P(
cm-’
1I ) transition.
(c, d)
HIGH-RESOLUTION
103
IR OF CYCLOPENTADIENE
11 times before detection and calibration against accurately measured CO* lines ( 10). However, due to the direct measurement of wavenumber provided by the FTIR spectrometer, the numerical results required for the analysis were taken from this source. Their estimated uncertainty is 0.001 cm-’ for well-resolved lines. ANALYSIS
The normal coordinate analysis of Castelluci et al. ( I I ) ascribes the 26 h fundamental to an in-phase displacement of the vinyl hydrogen atoms vibrating out of the carbon-
(a)
W
00
665.49
665.51
665.53
cm”
669.32
669.36
L 669.46 cm”
FIG. 3. Diode laser spectra of R-branch lines of pure CPD recorded in a jet: (a) R( 2) transition, (b) R( 9) transition. (c, d) Corresponding simulations.
104
BOARDMAN
ET AL.
ring plane and parallel to the c inertial axis [the inertial axis system is given in Ref. (4)]. CPD is a near-oblate top (K = 0.9) and the 26,!,band appears as a C-type (parallel) band with P, Q, and R branches (6). The asymmetric rotor selection rules are M = 0, fl;
AK, = +l (t3, *5, . . .);
AK,=O(+2,+4,
*se).
The most striking feature of this band at high resolution is the Q-branch region. Figure 1 shows the diode laser recording of the Q branch at room temperature and in the jet. The advantage of jet spectroscopy in revealing the lowest rotational states, which do not give rise to the strongest features at room temperature, is made immediately clear in the coldest spectrum (Fig. 1b) . The apparently quadratic dependence of the positions of the Q-branch features initially suggested that these were Kc = 1, 2, 3 . . components. This led to unacceptably large changes in the C rotational constant oLvibrationa1 excitation and the exact analysis proceeded using R- and P-branch lines starting at low N, where they were most easily observed with the diode laser (room temperature or jet cooled), and extending out to N = 30 in the FTIR spectrum. Representative P- and R-branch diode spectra are shown in Figs. 2 and 3, and part of the FTIR recording is in Fig. 4. A Watson A-reduced Hamiltonian, representation I’, was used for prediction and fitting purposes ( 12). Using the ground-state constants of Bogey and co-workers (5) and the position of the band center indicated in Fig. 1b, many components of each P- and R-branch transition (such as those shown in Figs. 2-4) were assigned and fitted. Only those lines not obviously merged with others in the FTIR spectrum were
676.20
676.45
676.70
676.95
FIG. 4. FHR spectrum of CPD, showing R(22), Torr pressure and with a resolution of 0.003 cm-‘.
R(23),
677.20 and R(24),
677.45 recorded
677.70cm-'
in a lkm-long
cell at 2
HIGH-RESOLUTION
IR OF CYCLOPENTADIENE TABLE I
Measured and Assigned P- and R-Branch Lines in the 26: Band of Cyclopentadiene
441331 4 3 1 551441 550 541431 661551 660 652 625 615 651541 771661 770 762 761651 881111 880
872 871761 991881 990 9 8 2 918 328 10 10 1 10 10 0 lc 9 2 10 5 6 10 4 6 10 2 9 1019 11 11 1 11 11 0 11 10 2 11 4 8 11 3 8 11 1 10 11 2 10 12 12 1 12 12 0 12 11 2 12 5 8 12 4 8 12 1 11 12 2 11 13 13 1 13 13 0 13 12 2 13 6 8 13 5 8 13 5 9 13 4 9 13 2 11 13 3 11 13 1 12 13 2 12 14 14 1 14 14 0 14 13 2 14 6 9 14 5 9 14 5 10 14 4 10 14 3 12 14 2 12 14 1 13 14 2 13 15 15 1 15 15 0 15 14 2 15 3 12 15 4 12
3
21
440 550 542 515 505
660 652 770
762
880 8 12 808 818 9 9 1 9 9 0 9 8 2 9 4 6 9 3 6 919 3 0 9 10 10 1 10 10 0 10 9 2 10 3 8 10 2 8 10 0 10 10 1 10 11 11 1 11 11 0 11 10 2 11 4 8 11 3 8 11 0 11 11 1 11 12 12 1 12 12 0 12 11 2 12 5 8 12 4 8 12 4 9 12 3 9 12 1 11 12 2 11 12 0 12 12 1 12 13 13 1 13 13 0 13 12 2 13 5 9 13 4 9 13 4 10 13 3 10 13 2 12 13 1 12 13 0 13 13 1 13 14 14 1 14 14 0 14 13 2 14 2 12 14 3 12
661.6064 -3 661.6351 -2 661.0428 -4 661.0575 6 661.0794 -3 660.4785 1 660.4935 3 660.5027 -3 660.5147 -1 660.5147 -1 660.5225 -4 659.9128 6 659.9271 -2 659.9411 2 659.9646 -3 659.3446 0 659.3595 4 659.3183 -4 659.4048 -6 658.1151 1 658.7889
0
658.8151 4 658.8494 3 658.8494 3 658.2052 0 658.2169 1 658.2492 -1 658.2137 2 658.2131 2 658.2344 4 658.2944 4 657.6334 0 657.6432 0 657.6823 -2 657.7221 -3 657.7221 -3 657.7381 -9 657.7381 -9 657.0603 1 657.0684 3 657.1143 0 657.1574 -5 657.1574 -5 657.1838 -3 657.1838 -3 656.4855 -2 656.4921 2 656.5449 3 656.5929 3 656.5329 3 656.5999 -3 656.5999 -3 656.6182 -4 656.6182 -4 656.6290 -3 656.6290 -3 655.9100 2 655.9152 6 655.9740 5 656.0337 -3 656.0337 -3 656.0425 -1 656.0425 -1 656.0628 -1 656.0628 -1 656.0744 -1 656.0744 -1 655.3333 8 655.3356 -6 655.4009 2 655.4966 9 655.4966 9
15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 17 17 17 17 17 17 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25
3 1 2 9 8 8 7 5 6 4 5 3 4 2 3 1 2 9 8 8 7 5 6 4 5 3 4 2 3 1 2 10 9 8 9 7 8 5 6 4 5 3 4 2 3 1 2 25 25 23 23 22 15 16 14 15 13 14 12 13 11 12 9 10 8 9 7 8 6 7 5 6 4 5 3
13 14 14 8 8 9 9 11 11 12 12 13 13 14 14 15 15 9 9 10 10 12 12 13 13 14 14 15 15 16 16 11 11 12 12 13 13 15 15 16 16 17 17 18 18 19 19 1 0 3 2 4 10 10 11 11 12 12 13 13 14 14 16 16 17 17 18 18 19 19 20 20 21 21 22
14 14 14 15 15 15 15 15 15 15 15 15 15 15 15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24
2 0 1 8 7 7 6 4 5 3 4 2 3 1 2 0 1 8 7 7 6 4 5 3 4 2 3 1 2 0 1 9 8 7 8 6 7 4 5 3 4 2 3 1 2 0 1 24 24 22 22 21 14 15 13 14 12 13 11 12 10 11 8 9 1
8 6 7 5 6 4 5 3 4 2
13 14 14 8 8 9 9 11 11 12 12 13 13 14 14 15 15 9 9 10 10 12 12 13 13 14 14 15 15 16 16 11 11 12 12 13 13 15 15 16 16 17 17 18 18 19 19 1 0 3 2 4 10 10 11 11 12 12 13 13 14 14 16 16 17 17 18 18 19 19 20 20 21 21 22
655.5081 8 655.5182 -17 655.5182 -17 654.8917 -4 654.8917 -4 654.8993 -1 654.8993 -1 654.9178 6 654.9178 6 654.9277 -1 654.9277 -1 654.9397 4 654.9397 4 654.9526 8 654.9526 8 654.9662 9 654.9662 9 654.3307 -2 654.3307 -2 654.3392 0 654.3392 0 654.3591 1 654.3591 1 654.3705 0 654.3705 0 654.3824 -6 654.3824 -6 654.3954 -0 654.3954 -0 654.4103 -6 654.4103 -6 652.6375 -4 652.6375 -4 652.6476 -5 652.6476 -5 652.6595 2 652.6595 2 652.6853 4 652.6853 4 652.6992 0 652.6392 0 652.7142 -3 652.7142 -3 652.7312 4 652.7312 4 652.7477 -3 652.7477 -3 649.4333 -1 649.4933 -2 649.6789 -7 649.7146 2 649.7360 3 649.7638 -1 649.7638 -1 649.7722 3 649.7722 3 649.7814 1 649.7814 1 649.7921 1 649.7921 1 >49.8038 0 649.8038 0 649.8307 0 649.8307 0 649.8454 -3 649.8454 -3 649.8616 -1 649.8616 -1 649.8785 -2 649.8785 -2 649.8965 -2 643.8965 -2 649.9158 1 649.9158 1 649.9350 -6
105
106
BOARDMAN
ET AL.
TABLE I-Continued Observed G 25 25 25 25 25 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
22 23 23 24 24 3 2 4 11 13 14 15 16 5 17 18 4 19 20 21 22 23 24 25 26 27 28
N
Y
24 24 24 24 24 29 23 29 23 29 23 23 23 29 23 29 29 29 23 23 23 29 29 23 29 23 23
3 1 2 0 1 27 27 26 18 16 15 14 13 24 12 11 25 10 3 8 7 6 5 4 3 2 1
K,
; cm-'
ObsCalC Xl@V
22 23 23 24 24 3 2 4 11 13 14 15 16 5 17 18 4 19 20 21 22 23 24 25 26 27 28
649.9350 643.3565 649.9565 643.3786 649.3786 646.7418 646.7535 646.8285 646.8881 646.9061 646.9170 646.3236 646.9430 646.9501 646.9575 646.9733 646.9809 646.9901 647.0073 647.0259 647.0457 647.0662 647.0887 647.1100 647.1342 647.1591 647.1044
-6 -1 -1 1 1 0 -6 2 -2 -1 -3 1 0 -4 0 2 1 3 -2 -3 -2 -4 4 -9 -4 -1 -4
2 5 5 6 6 3 2 1 3 6 6 7 7 10 10 2 1 3 6 6 7 7 8 8 10 10 11 11 2 1 9 9
665.5006 665.5186 666.0507 666.0737 666.5990 666.6279 667.1478 667.1581 667.1810 667.6363 668.2438 668.2198 668.2837 668.7923 668.8354 669.3381 663.3645 669.3645 663.3636 669.3636 669.3920 669.8801 663.8854 663.8367 669.9155 669.9155 669.9221 669.9221 663.3457 663.9457 670.4222 670.4327 670.4384 670.4603 670.4603 670.4666 670.4666 670.4749 670.4749 670.4913 670.4913 670.5018 670.5018 670.9641 670.9799 671.0265 671.0265
0 -2 6 0 -2 1 1 -1 0 4 1 0 -11 10 0 10 1 1 -5 -5 0 3 -4 4 0 0 1 1 -11 -11 1 -2 -3 3 3 0 0 3 9 -2 -2 2 2 -1 2 4 4
R Branch. 211321 221331 321431 331441 431541 441551 541651 532 551661 651761 761871 752 771881 871981 881991 9 7 9 4 3 5 9 3 3 4 919 10 8 10 3 10 7 10 4 10 5 10 3 10 4 1 10 10 0 11 9 11 10 11 8 11 5 11 6 11 4 11 5 11 3 11 4 11 1 11 2 11 0 11 1 12 10 12 11 12 3 :2 4
642
862
2 5 5 6 6 2 1 3 6 6 7 7 10 10 2 1 3 6 6 7 7 8 8 10 10 11 11 2 1 9 9
10 10 10 10 10 10 11 11 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12 12 12 12 12 12 13 13 13 13
8 5 6 4 5 2 3 10 8 5 6 4 5 2 1 10 11 3 6 7 5 6 4 5 2 3 1 2 11 12 4 5
aI-'
Observed N'
I(IK:
12 2 12 3 12 1 12 2 12 0 12 1 13 11 13 10 13 12 13 3 13 4 3 13 2 13 13 1 13 2 13 0 13 1 14 12 14 11 14 3 4 14 14 3 14 2 14 1 14 2 14 0 14 1 15 13 15 12 15 9 15 10 15 7 15 8 15 6 15 7 15 4 15 5 15 3 15 4 15 2 15 3 1 15 15 2 15 0 15 1 16 8 16 3 16 7 16 8 16 5 6 16 16 4 16 5 16 3 16 4 16 2 16 3 16 1 16 2 16 0 1 16 17 13 17 9 17 10 17 6 17 3 17 7 17 8 17 17 17 6 17 7 17 6 17 5 17 4 17 5 4 17 17 3
10 10 11 11 12 12 2 3 1 10 10 11 11 12 12 13 13 2 3 11 11 12 12 13 13 14 14 2 3 6 6 8 8 3 3 11 11 12 12 13 13 14 14 15 15 8 8 3 3 11 11 12 12 13 13 14 14 15 15 16 16 4 8 8 9 3 10 10 0 11 11 12 12 13 13 14 14
N
Y
Kc
13 13 13 13 13 13 14 14 14 14 14 14 14 14 14 14 14 15 15 15 15 15 15 15 15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
3 4 2 3 1 2 12 11 13 4 5 4 3 2 3 1 2 13 12 4 5 4 3 2 3 1 2 14 13 10 11 8 3 7 8 5 6 4 5 3 4 2 3 1 2 3 10 8 3 6 7 5 6 4 5 3 4 2 3 1 2 14 10 11 3 10 8 9 18 7 8 7 6 5 6 5 4
10 10 11 11 12 12 2 3 1 10 10 11 11 12 12 13 13 2 3 11 11 12 12 13 13 14 14 2 3 6 6 8 8 3 3 11 11 12 12 13 13 14 14 15 15 8 8 3 3 11 11 12 12 13 13 14 14 15 15 16 16 4 8 8 3 3 10 10 0 11 11 12 12 13 13 14 14
; cm., 671.0356 671.0356 671.0457 671.0457 671.0568 671.0568 671.5061 671.5197 671.5264 671.5780 671.5780 671.5885 671.5885 671.5995 671.5995 671.6116 671.6116 672.0482 672.0588 672.1306 672.1306 672.1416 672.1416 672.1536 672.1536 672.1670 672.1670 672.5893 672.5966 672.6232 672.6232 672.6432 672.6432 672.6520 672.6520 672.6710 672.6710 672.6829 672.6829 672.6942 672.6942 672.7078 672.7078 672.7217 672.7217 673.1832 673.1832 673.1919 673.1919 673.2117 673.2117 673.2233 673.2233 673.2351 673.2351 673.2482 673.2482 673.2618 673.2618 673.2765 673.2765 673.6893 673.7226 673.7226 673.7310 673.7310 673.7408 673.7408 673.7445 673.7512 673.7512 673.7624 673.7624 673.7742 673.7742 673.7875 673.7875
ObSCaiC XIV' 3 3 2 2 2 2 1 0 1 -2 -2 1 1 0 0 0 0 5 3 2 2 0 0 0 0 4 4 1 2 4 4 -3 -3 0 0 -5 -5 2 2 -6 -6 0 0 -1 -1 -3 -3 -2 -2 0 0 4 4 1 1 2 2 -2 -2 -5 -5 -2 -1 -1 -3 -3 1 1 0 2 2 2 2 -1 -1 1 1
an-
HIGH-RESOLUTION
107
IR OF CYCLOPENTADIENE
TABLE I-Continued Observed " cm-' 17 17 17 17 17 17 18 18 18 18 18 18 18 18 18 18 18 18 18 18 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
2 3 2 1 0 1 6 7 5 6 4 5 3 4 2 3 1 2 0 1 16 17 15 12 13 10 11 9 10 7 8 6 7 5 6 4 5 3 4 2 3 1 2 0 1 11 12 10 11 8 9 5 6 4 5 3 4 2 3 1 2
15 15 16 16 17 17 12 12 13 13 14 14 15 15 16 16 17 17 18 18 3 2 4 1
7 9 9 10 10 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 9 9 10 10 12 12 15 15 16 16 17 17 18 18 19 19
18 18 18 18 18 18 19 19 19 19 19 19 19 19 19 19 19 19 19 19 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21
3 4 3 2 1 2 7 8 6 7 5 6 4 5 3 4 2 3 1 2 17 18 16 13 14 11 12 10 11 8 9 7 8 6 7 5 6 4 5 3 4 2 3 1 2 12 13 11 12 9 10 6 7 5 6 4 5 3 4 2 3
15 15 16 16 17 17 12 12 13 13 14 14 15 15 16 16 17 17 18 18 3 2 4 7 7 9 9 10 10 12 12 13 13 14 14 15 15 16 16 17 :7 18 18 19 19 9 9 10 10 12 12 15 15 16 16 17 17 18 18 19 19
673.8007 673.8007 673.8161 673.8161 673.8322 673.8322 674.3010 674.3010 674.3130 674.3130 674.3261 674.3261 674.3396 674.3396 674.3553 674.3553 674.3707 674.3707 674.3878 674.3878 674.7427 674.7527 674.7608 674.7900 674.7900 674.8071 674.8071 674.8165 674.8165 674.8380 674.8380 674.8499 674.8499 674.8632 674.8632 674.8771 674.8771 674.8926 674.8926 674.9081 674.9081 674.9259 674.9259 674.9437 674.9437 675.3433 675.3433 675.3528 675.3528 675.3741 675.3741 675.4138 675.4138 675.4284 675.4284 675.4453 675.4453 675.4619 675.4619 675.4808 675.4808
ObscalC Xl@V -7 -7 -2 -2 -1 -1 4 4 3 3 3 3 -2 -2 5 5 0 0 2 2 -3 -7 2 -2 -2 2 2 1 1 -1 -1 -3 -3 -1 -1 -3 -3 3 3 -2 -2 7 7 7 7 -1 -1 -1 -1 -5 -5 -2 -2 -6 -6 4 4 1 1 10 10
_~
L-n-'
Observed N 20 20 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
20 20 4 3 5 8 8 10 10 12 12 13 13 15 15 16 16 17 17 4 5 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29
obsxalc
N
Y
K,
; ",,-I
xl@;
21 21 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31
1 2 23 24 22 19 20 17 18 15 16 14 15 12 13 11 12 10 11 27 26 20 21 19 20 18 19 17 18 17 17 15 16 14 15 13 14 12 13 11 12 10 11 9 10 8 9 7 8 6 7 5 6 4 5 3 4 2 3
20 20 4 3 5 8 8 10 10 12 12 13 13 15 15 16 16 17 17 4 5 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29
675.4994 675.4994 678.4842 678.4899 678.5017 678.5336 678.5336 678.5529 678.5529 678.5750 678.5750 678.5871 678.5871 678.6145 678.6145 678.6296 678.6296 678.6455 678.6455 680.6046 680.6107 680.6782 680.6782 680.6896 680.6896 680.7025 680.7025 680.7158 680.7158 680.7292 680.7292 680.7447 680.7447 680.7614 680.7614 680.7775 680.7775 680.7959 680.7959 680.8146 680.8146 680.8347 680.8347 680.8549 680.8549 680.8771 680.8771 680.8992 680.8992 680.9232 680.9232 680.9480 680.9480 680.9744 680.9744 681.0009 681.0009 681.0289 681.0289
9 9 -2 -3 2 0 0 3 3 3 3 1 1 1 1 2 2 0 0 3 -5 0 0 -1 -1 4 4 4 4 -4 -4 0 0 7 7 -2 -2 3 3 1 1 3 3 -3 -3 1 1 -6 -6 -4 -4 -3 -3 4 4 2 2 5 5
~msErr.x
0.000383
cm-'
on-'
included in the fit but later simulations were able to reproduce the merged lines with high accuracy. The more complicated patterns at higher N were assigned with the help of preliminary simulations including relevant nuclear spin statistics for intensities. Assuming a planar carbon-ring structure, CPD contains three pairs of protons in which each proton interchanges with its partner under the C2 operation of the Czv
108
BOARDMAN ET AL.
point group. Since for the exchange of an odd number of fermion pairs, the total wavefunction must be antisymmetric, each rotational level is given a weight thus:
e
e
0
0
e
0
0
e
Symmetry species in C,
Weight
A A B B
7 7 9 9
This manifests itself most obviously in the Kc = 1 (outer) components of the low-N P- and R-branch transitions. Generally, in R (N = even) transitions the lowerwavenumber Kc component (with the lower of the two possible K, values) has lower intensity than the higher-wavenumber component. In R (N = odd) transitions the intensity ratio reverses. For P (N = even) transitions the lower-wavenumber Kc = 1 component has the greater intensity. These effects may be seen in Figs. 2 and 3. In a final fit of the 420 P- and R-branch lines given in Table I, the ground-state parameters were fixed to the microwave spectrum values (5). The upper state parameters resulting from the least-squares fit are given in Table II and the %,bs_calc in Table I. Although the Q-branch features were not included in the fit, they were used to test the final parameters by simulating the room temperature and jet spectra. The Qbranch lines shown in Fig. 1 are blue shaded and both the regular structure on the
TABLE II Molecular Parameters (cm-‘) for the Ground-State and 26, Levels of Cyclopentadiene ( 1c Uncertainties Given in Parentheses) Ground State.(a)
Level 26,
A
0.28106478
(7)
0.2804694
(11)
B
0.27437782
(7)
0.2740983
(10)
C
0.14247981(l)
0.1425197
(3)
108,
AN
5.15(l)
5.178 (62)
108,
AM
-2.04(4)
-1.80 (25)
108,
AK
5.61(3)
5.09 (35)
log,6,
2.04 (5)
2.083(33)
108,
2.955(9)
2.81(10)
6,
Go (26;)
663.84800(5)
(a) fixed at the values from the mm wave spectrum (ref. 5)
HIGH-RESOLUTION
IR OF CYCLOPENTADIENE
109
high-wavenumber side and irregular lines on the low-wavenumber side in Fig. la (not visible in the lower S:N ratio FTIR spectra) are definite spectroscopic features due to high rotational transitions which are suppressed in the colder jet spectra. The appropriate simulations with assumed Doppler profiles of 0.002 cm-’ FWHM are shown in Figs. lc and d. Representative simulations of the P and R branches are shown in Figs. 2 and 3 (c and d). One further feature of the room temperature FTIR spectrum should be noted: a closely similar, but weaker Q branch was detected between 664.1 and 664.2 cm-‘. DISCUSSION
The results of the present study not only consolidate the earlier assignment of this band to a vibration of B2 species but also confirm the value of 664 cm-’ for the band center. The position of the band origin predicted from the fit, Go= 663.848 cm-‘, is in excellent agreement with the narrow, intense feature at 663.847 cm-’ in the coldest jet spectrum (Fig. 1b) . This linelike feature is an accumulation of lines with K, = N. At low temperatures only a few low-N lines contribute to its intensity but at higher temperatures many higher N levels are populated, giving rise to the blue shading in Fig. 1a. Lower-wavenumber Q-branch features correspond to aggregations of transitions with K, = N - 1, Kc = N - 2, etc. The Q-branch feature, resembling the fundamental at 664.2 cm-‘, can be tentatively assigned to the 26627 f hot band. The simulation of the Q branch reproduces not only the intensity and shadings of the main features but also the detailed fine structure in the room temperature spectrum. This includes the broad rising background extending to lower wavenumber, which disappears in the cold spectrum. Simulations of the lowest-temperature spectra (Fig. 1d) showed that although their rotational distributions were not completely thermalized, they corresponded to T,, N 25 K. In the P and R branches, the Kc patterns of each P and R transition are relatively insensitive to temperature. In addition, only one P or R transition appears on a single laser mode. A quantitative derivation of T,, from them is therefore not readily achieved. In conclusion, the planarity of the carbon ring is confirmed by the present study. There are no significant changes in the inertial defect or centrifugal distortion constants on excitation of this mode, which might have been expected from comparison with other planar molecules manifesting nonrigidity. This mode of CPD seems free of any obvious perturbations and the successful analysis of its IR spectrum supports the symmetry assigned to this mode in both the low-resolution vibrational and force-field analyses. ACKNOWLEDGMENTS. We thank the SERC for a studentship for S.R.B. and Unilever Research, Port Sunlight studentship for S.A.B. in addition to several equipment grants. We also thank Professor the loan of equipment. REFERENCES 1. A. C. LEGON, Chem. Rev. 80,231-262 (1980). 2. V. W. LAURIE, J. Chem. Phys. 24635-636 (1956).
Laboratory, for a B. A. Thrush for
110
BOARDMAN
ET AL.
3. L. H. S~HARPENAND V. W. LAURIE,J. Chem. Phys. 43,2X5-2166 ( 1965). 4. D. DAMIANI, L. FENETTI,AND E. GALLINELLA,Chem. Phys. Lett. 37,265-269 (1976). 5. M. BOGEY, C. DEMUYNCK,AND J. L. DESTOMBES, J. Mol. Spectrosc. 132,277-279 ( 1988). 6. E. GALLINELLA,B. FORTUNATO,AND P. MIRONE,J. Mol. Spectrosc. 24,345-362 ( 1967). 7. K. B. THAKUR, V. A. JOB, AND V. B. KARTHA,J. Mol. Spectrosc. 112,340-346 ( 1985). 8. G. MAGNCJSSON, J. Org. Chem. SO, 1998 (1985). 9. P. B. DAVIES,N. A. MARTIN, AND M. D. NUNES, Spectrochim. Acta A 45,293-298 ( 1989). IO. G. GUEIXHVILI AND K. NARAHATIRAO, “Handbook of Infrared Standards,” Academic Press, Orlando, FLy 1986. 11. E. CASTELLUCI,P. MANZELLI,B. FORTUNATO,E. GALLINELLA,AND P. MIRONE, Spectrochim. Acta A 31,451-461 (1975). 12. J. K. G. WATSON, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 6, pp. 2-89, Elsevier, Amsterdam, 1977.
OF MOLECULAR
SPECTROSCOPY
143, IOO- 110 ( 1990)
High-Resolution FTIR and Diode Laser Jet Spectroscopy of Cyclopentadiene STEVEN R. BOARDMAN,
STEWART A. BONE, PAUL B. DAVIES,
AND NICHOLAS
A. MARTIN
Department of Chemistry, University of Cambridge, LensJield Road, Cambridge CB2 1E W, United Kingdom High-resolution (Doppler-limited) infrared absorption spectra of the 26 h band of cyclopentadiene ( C5H6) have been recorded with FfIR and diode lasers. In addition to room temperature measurements, diode laser spectra were also recorded in a molecular jet. The latter experiments were particularly useful in revealing the low-N transitions in the Q branch. Analysis of 420 P- and Rbranch lines of this C-type band, with the ground-state constants fixed at microwave-determined values, yielded rotation and distortion parameters for the upper (26,) level. The derived band origin, Z0 = 663.84800( 5) cm-r, is in good agreement with that from the vibrational analysis at lower KSOhtiOn. 0 1990 Academic Press. Inc. INTRODUCTION
Cyclopentadiene (CPD) and its derivatives have played an important part in organometallic and structural chemistry. It is the prototype molecule for many other five-membered rings containing two double bonds, including heterocycles, which have been widely studied by electron diffraction and microwave spectroscopy. In the latter case the derived spectroscopic parameters have been linked to the potential energy surface and hence the conformation of the molecule ( 1). Cyclopentadiene itself has been extensively studied by microwave spectroscopy (2-5) culminating in the complete determination of its r, and r. structures by Damiani and co-workers (4). They used several deuterated CPD isomers and the earlier structural data of Scharpen and Laurie (3) to achieve this. More recently, measurements were extended into the millimeter region for astrophysical purposes (5) following the discovery of other small ring molecules in the interstellar medium. In contrast to the numerous microwave studies, considerably less has been reported on the infrared spectroscopy of CPD. In the gas phase, IR studies have been restricted to a vibrational band analysis, which in conjunction with Raman spectroscopy led to the assignment of the 27 fundamentals (6). The IR spectra of the vapor recorded by Gallinella et al. (6) showed considerable band overlap above ca. 700 cm-‘. However, at lower wavenumbers, two fundamentals, the ring bending mode at 350 cm-’ (27 A, B2)and the CH out-of-plane bending mode of the vinyl hydrogen atoms at 664 cm-’ (26;, B2), w ere well resolved. This article reports a study of the latter fundamental using FTIR at room temperature, with complementary diode laser jet spectroscopy to simplify regions where poor signal-to-noise ratios or spectral congestion made the FIIR spectrum less useful. The jet spectra also provided a rigorous test of the derived spectroscopic parameters which were used to simulate representative regions of the 0022-2852190 $3.00 Copyright 0
1990 by Academic Press, Inc.
All rights of reprcduction in any form reserved.
100
HIGH-RESOLUTION
101
IR OF CYCLOPENTADIENE
diode spectra. In addition to providing the first rotationally resolved IR spectrum of CPD, this study is of general relevance for extending IR spectroscopy to near-oblate tops for which very few high-resolution studies have been reported ( 7). EXPERIMENTAL
DETAILS
Cyclopentadiene was prepared by the thermolysis of the dimer (dicyclopentadiene, Aldrich, 95% purity) over copper (8) and its purity checked by low-resolution FUR. On the time scale of the experiments reported here ( 13 h in the case of some FTIR spectral recordings) no significant dimerization occurred. FTIR spectra were recorded at an unapodized resolution of approximately 0.003 cm-’ on a BOMEM DA3.002
(a)
(d)
cc: 663.7
663.8
663.9
cm-’
663.7
663.8
,‘.
663.9
.
.
.
(
cm-’
FIG. 1. Diode laser spectra of the Q-branch region of the 26; mode of cyclopentadiene. (a) Room temperature spectrum in a 1km cell at 2 mm Hg pressure. (b) Jet-cooled spectrum using 5% CPD in Ar. (c, d) Corresponding simulations using the parameters in Table II at temperatures 7’,, = 298 and 25 K. respectively.
102
BOARDMAN
ET AL.
spectrometer using a Si:B detector and a KBr beam splitter, along with a 15-cm-long single-pass cell filled to a pressure of0.2-2 Torr. Room temperature diode iaser spectra were recorded under similar conditions. The experimental arrangement for diode laser jet spectroscopy has been described elsewhere (9). Pure CPD or mixtures with He or Ar at stagnation pressures of up to 2 atm were expanded into the vacuum chamber via a Newport Corporation pulsed valve ( 192 Hz, l-mm-diameter orifice). The diode laser beam traversed the jet up to
(d)
h 660.47
660.49
660.51
FIG. 2. Diode laser spectra Corresponding simulations.
660.53
of jet-cooled
cm”
657.62
pure CPD: (a) P(6)
657.67 transition,
657.72 (b) P(
cm-’
1I ) transition.
(c, d)
HIGH-RESOLUTION
103
IR OF CYCLOPENTADIENE
11 times before detection and calibration against accurately measured CO* lines ( 10). However, due to the direct measurement of wavenumber provided by the FTIR spectrometer, the numerical results required for the analysis were taken from this source. Their estimated uncertainty is 0.001 cm-’ for well-resolved lines. ANALYSIS
The normal coordinate analysis of Castelluci et al. ( I I ) ascribes the 26 h fundamental to an in-phase displacement of the vinyl hydrogen atoms vibrating out of the carbon-
(a)
W
00
665.49
665.51
665.53
cm”
669.32
669.36
L 669.46 cm”
FIG. 3. Diode laser spectra of R-branch lines of pure CPD recorded in a jet: (a) R( 2) transition, (b) R( 9) transition. (c, d) Corresponding simulations.
104
BOARDMAN
ET AL.
ring plane and parallel to the c inertial axis [the inertial axis system is given in Ref. (4)]. CPD is a near-oblate top (K = 0.9) and the 26,!,band appears as a C-type (parallel) band with P, Q, and R branches (6). The asymmetric rotor selection rules are M = 0, fl;
AK, = +l (t3, *5, . . .);
AK,=O(+2,+4,
*se).
The most striking feature of this band at high resolution is the Q-branch region. Figure 1 shows the diode laser recording of the Q branch at room temperature and in the jet. The advantage of jet spectroscopy in revealing the lowest rotational states, which do not give rise to the strongest features at room temperature, is made immediately clear in the coldest spectrum (Fig. 1b) . The apparently quadratic dependence of the positions of the Q-branch features initially suggested that these were Kc = 1, 2, 3 . . components. This led to unacceptably large changes in the C rotational constant oLvibrationa1 excitation and the exact analysis proceeded using R- and P-branch lines starting at low N, where they were most easily observed with the diode laser (room temperature or jet cooled), and extending out to N = 30 in the FTIR spectrum. Representative P- and R-branch diode spectra are shown in Figs. 2 and 3, and part of the FTIR recording is in Fig. 4. A Watson A-reduced Hamiltonian, representation I’, was used for prediction and fitting purposes ( 12). Using the ground-state constants of Bogey and co-workers (5) and the position of the band center indicated in Fig. 1b, many components of each P- and R-branch transition (such as those shown in Figs. 2-4) were assigned and fitted. Only those lines not obviously merged with others in the FTIR spectrum were
676.20
676.45
676.70
676.95
FIG. 4. FHR spectrum of CPD, showing R(22), Torr pressure and with a resolution of 0.003 cm-‘.
R(23),
677.20 and R(24),
677.45 recorded
677.70cm-'
in a lkm-long
cell at 2
HIGH-RESOLUTION
IR OF CYCLOPENTADIENE TABLE I
Measured and Assigned P- and R-Branch Lines in the 26: Band of Cyclopentadiene
441331 4 3 1 551441 550 541431 661551 660 652 625 615 651541 771661 770 762 761651 881111 880
872 871761 991881 990 9 8 2 918 328 10 10 1 10 10 0 lc 9 2 10 5 6 10 4 6 10 2 9 1019 11 11 1 11 11 0 11 10 2 11 4 8 11 3 8 11 1 10 11 2 10 12 12 1 12 12 0 12 11 2 12 5 8 12 4 8 12 1 11 12 2 11 13 13 1 13 13 0 13 12 2 13 6 8 13 5 8 13 5 9 13 4 9 13 2 11 13 3 11 13 1 12 13 2 12 14 14 1 14 14 0 14 13 2 14 6 9 14 5 9 14 5 10 14 4 10 14 3 12 14 2 12 14 1 13 14 2 13 15 15 1 15 15 0 15 14 2 15 3 12 15 4 12
3
21
440 550 542 515 505
660 652 770
762
880 8 12 808 818 9 9 1 9 9 0 9 8 2 9 4 6 9 3 6 919 3 0 9 10 10 1 10 10 0 10 9 2 10 3 8 10 2 8 10 0 10 10 1 10 11 11 1 11 11 0 11 10 2 11 4 8 11 3 8 11 0 11 11 1 11 12 12 1 12 12 0 12 11 2 12 5 8 12 4 8 12 4 9 12 3 9 12 1 11 12 2 11 12 0 12 12 1 12 13 13 1 13 13 0 13 12 2 13 5 9 13 4 9 13 4 10 13 3 10 13 2 12 13 1 12 13 0 13 13 1 13 14 14 1 14 14 0 14 13 2 14 2 12 14 3 12
661.6064 -3 661.6351 -2 661.0428 -4 661.0575 6 661.0794 -3 660.4785 1 660.4935 3 660.5027 -3 660.5147 -1 660.5147 -1 660.5225 -4 659.9128 6 659.9271 -2 659.9411 2 659.9646 -3 659.3446 0 659.3595 4 659.3183 -4 659.4048 -6 658.1151 1 658.7889
0
658.8151 4 658.8494 3 658.8494 3 658.2052 0 658.2169 1 658.2492 -1 658.2137 2 658.2131 2 658.2344 4 658.2944 4 657.6334 0 657.6432 0 657.6823 -2 657.7221 -3 657.7221 -3 657.7381 -9 657.7381 -9 657.0603 1 657.0684 3 657.1143 0 657.1574 -5 657.1574 -5 657.1838 -3 657.1838 -3 656.4855 -2 656.4921 2 656.5449 3 656.5929 3 656.5329 3 656.5999 -3 656.5999 -3 656.6182 -4 656.6182 -4 656.6290 -3 656.6290 -3 655.9100 2 655.9152 6 655.9740 5 656.0337 -3 656.0337 -3 656.0425 -1 656.0425 -1 656.0628 -1 656.0628 -1 656.0744 -1 656.0744 -1 655.3333 8 655.3356 -6 655.4009 2 655.4966 9 655.4966 9
15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 17 17 17 17 17 17 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25 25
3 1 2 9 8 8 7 5 6 4 5 3 4 2 3 1 2 9 8 8 7 5 6 4 5 3 4 2 3 1 2 10 9 8 9 7 8 5 6 4 5 3 4 2 3 1 2 25 25 23 23 22 15 16 14 15 13 14 12 13 11 12 9 10 8 9 7 8 6 7 5 6 4 5 3
13 14 14 8 8 9 9 11 11 12 12 13 13 14 14 15 15 9 9 10 10 12 12 13 13 14 14 15 15 16 16 11 11 12 12 13 13 15 15 16 16 17 17 18 18 19 19 1 0 3 2 4 10 10 11 11 12 12 13 13 14 14 16 16 17 17 18 18 19 19 20 20 21 21 22
14 14 14 15 15 15 15 15 15 15 15 15 15 15 15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24 24
2 0 1 8 7 7 6 4 5 3 4 2 3 1 2 0 1 8 7 7 6 4 5 3 4 2 3 1 2 0 1 9 8 7 8 6 7 4 5 3 4 2 3 1 2 0 1 24 24 22 22 21 14 15 13 14 12 13 11 12 10 11 8 9 1
8 6 7 5 6 4 5 3 4 2
13 14 14 8 8 9 9 11 11 12 12 13 13 14 14 15 15 9 9 10 10 12 12 13 13 14 14 15 15 16 16 11 11 12 12 13 13 15 15 16 16 17 17 18 18 19 19 1 0 3 2 4 10 10 11 11 12 12 13 13 14 14 16 16 17 17 18 18 19 19 20 20 21 21 22
655.5081 8 655.5182 -17 655.5182 -17 654.8917 -4 654.8917 -4 654.8993 -1 654.8993 -1 654.9178 6 654.9178 6 654.9277 -1 654.9277 -1 654.9397 4 654.9397 4 654.9526 8 654.9526 8 654.9662 9 654.9662 9 654.3307 -2 654.3307 -2 654.3392 0 654.3392 0 654.3591 1 654.3591 1 654.3705 0 654.3705 0 654.3824 -6 654.3824 -6 654.3954 -0 654.3954 -0 654.4103 -6 654.4103 -6 652.6375 -4 652.6375 -4 652.6476 -5 652.6476 -5 652.6595 2 652.6595 2 652.6853 4 652.6853 4 652.6992 0 652.6392 0 652.7142 -3 652.7142 -3 652.7312 4 652.7312 4 652.7477 -3 652.7477 -3 649.4333 -1 649.4933 -2 649.6789 -7 649.7146 2 649.7360 3 649.7638 -1 649.7638 -1 649.7722 3 649.7722 3 649.7814 1 649.7814 1 649.7921 1 649.7921 1 >49.8038 0 649.8038 0 649.8307 0 649.8307 0 649.8454 -3 649.8454 -3 649.8616 -1 649.8616 -1 649.8785 -2 649.8785 -2 649.8965 -2 643.8965 -2 649.9158 1 649.9158 1 649.9350 -6
105
106
BOARDMAN
ET AL.
TABLE I-Continued Observed G 25 25 25 25 25 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
22 23 23 24 24 3 2 4 11 13 14 15 16 5 17 18 4 19 20 21 22 23 24 25 26 27 28
N
Y
24 24 24 24 24 29 23 29 23 29 23 23 23 29 23 29 29 29 23 23 23 29 29 23 29 23 23
3 1 2 0 1 27 27 26 18 16 15 14 13 24 12 11 25 10 3 8 7 6 5 4 3 2 1
K,
; cm-'
ObsCalC Xl@V
22 23 23 24 24 3 2 4 11 13 14 15 16 5 17 18 4 19 20 21 22 23 24 25 26 27 28
649.9350 643.3565 649.9565 643.3786 649.3786 646.7418 646.7535 646.8285 646.8881 646.9061 646.9170 646.3236 646.9430 646.9501 646.9575 646.9733 646.9809 646.9901 647.0073 647.0259 647.0457 647.0662 647.0887 647.1100 647.1342 647.1591 647.1044
-6 -1 -1 1 1 0 -6 2 -2 -1 -3 1 0 -4 0 2 1 3 -2 -3 -2 -4 4 -9 -4 -1 -4
2 5 5 6 6 3 2 1 3 6 6 7 7 10 10 2 1 3 6 6 7 7 8 8 10 10 11 11 2 1 9 9
665.5006 665.5186 666.0507 666.0737 666.5990 666.6279 667.1478 667.1581 667.1810 667.6363 668.2438 668.2198 668.2837 668.7923 668.8354 669.3381 663.3645 669.3645 663.3636 669.3636 669.3920 669.8801 663.8854 663.8367 669.9155 669.9155 669.9221 669.9221 663.3457 663.9457 670.4222 670.4327 670.4384 670.4603 670.4603 670.4666 670.4666 670.4749 670.4749 670.4913 670.4913 670.5018 670.5018 670.9641 670.9799 671.0265 671.0265
0 -2 6 0 -2 1 1 -1 0 4 1 0 -11 10 0 10 1 1 -5 -5 0 3 -4 4 0 0 1 1 -11 -11 1 -2 -3 3 3 0 0 3 9 -2 -2 2 2 -1 2 4 4
R Branch. 211321 221331 321431 331441 431541 441551 541651 532 551661 651761 761871 752 771881 871981 881991 9 7 9 4 3 5 9 3 3 4 919 10 8 10 3 10 7 10 4 10 5 10 3 10 4 1 10 10 0 11 9 11 10 11 8 11 5 11 6 11 4 11 5 11 3 11 4 11 1 11 2 11 0 11 1 12 10 12 11 12 3 :2 4
642
862
2 5 5 6 6 2 1 3 6 6 7 7 10 10 2 1 3 6 6 7 7 8 8 10 10 11 11 2 1 9 9
10 10 10 10 10 10 11 11 11 11 11 11 11 11 11 12 12 12 12 12 12 12 12 12 12 12 12 12 13 13 13 13
8 5 6 4 5 2 3 10 8 5 6 4 5 2 1 10 11 3 6 7 5 6 4 5 2 3 1 2 11 12 4 5
aI-'
Observed N'
I(IK:
12 2 12 3 12 1 12 2 12 0 12 1 13 11 13 10 13 12 13 3 13 4 3 13 2 13 13 1 13 2 13 0 13 1 14 12 14 11 14 3 4 14 14 3 14 2 14 1 14 2 14 0 14 1 15 13 15 12 15 9 15 10 15 7 15 8 15 6 15 7 15 4 15 5 15 3 15 4 15 2 15 3 1 15 15 2 15 0 15 1 16 8 16 3 16 7 16 8 16 5 6 16 16 4 16 5 16 3 16 4 16 2 16 3 16 1 16 2 16 0 1 16 17 13 17 9 17 10 17 6 17 3 17 7 17 8 17 17 17 6 17 7 17 6 17 5 17 4 17 5 4 17 17 3
10 10 11 11 12 12 2 3 1 10 10 11 11 12 12 13 13 2 3 11 11 12 12 13 13 14 14 2 3 6 6 8 8 3 3 11 11 12 12 13 13 14 14 15 15 8 8 3 3 11 11 12 12 13 13 14 14 15 15 16 16 4 8 8 9 3 10 10 0 11 11 12 12 13 13 14 14
N
Y
Kc
13 13 13 13 13 13 14 14 14 14 14 14 14 14 14 14 14 15 15 15 15 15 15 15 15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 17 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18 18
3 4 2 3 1 2 12 11 13 4 5 4 3 2 3 1 2 13 12 4 5 4 3 2 3 1 2 14 13 10 11 8 3 7 8 5 6 4 5 3 4 2 3 1 2 3 10 8 3 6 7 5 6 4 5 3 4 2 3 1 2 14 10 11 3 10 8 9 18 7 8 7 6 5 6 5 4
10 10 11 11 12 12 2 3 1 10 10 11 11 12 12 13 13 2 3 11 11 12 12 13 13 14 14 2 3 6 6 8 8 3 3 11 11 12 12 13 13 14 14 15 15 8 8 3 3 11 11 12 12 13 13 14 14 15 15 16 16 4 8 8 3 3 10 10 0 11 11 12 12 13 13 14 14
; cm., 671.0356 671.0356 671.0457 671.0457 671.0568 671.0568 671.5061 671.5197 671.5264 671.5780 671.5780 671.5885 671.5885 671.5995 671.5995 671.6116 671.6116 672.0482 672.0588 672.1306 672.1306 672.1416 672.1416 672.1536 672.1536 672.1670 672.1670 672.5893 672.5966 672.6232 672.6232 672.6432 672.6432 672.6520 672.6520 672.6710 672.6710 672.6829 672.6829 672.6942 672.6942 672.7078 672.7078 672.7217 672.7217 673.1832 673.1832 673.1919 673.1919 673.2117 673.2117 673.2233 673.2233 673.2351 673.2351 673.2482 673.2482 673.2618 673.2618 673.2765 673.2765 673.6893 673.7226 673.7226 673.7310 673.7310 673.7408 673.7408 673.7445 673.7512 673.7512 673.7624 673.7624 673.7742 673.7742 673.7875 673.7875
ObSCaiC XIV' 3 3 2 2 2 2 1 0 1 -2 -2 1 1 0 0 0 0 5 3 2 2 0 0 0 0 4 4 1 2 4 4 -3 -3 0 0 -5 -5 2 2 -6 -6 0 0 -1 -1 -3 -3 -2 -2 0 0 4 4 1 1 2 2 -2 -2 -5 -5 -2 -1 -1 -3 -3 1 1 0 2 2 2 2 -1 -1 1 1
an-
HIGH-RESOLUTION
107
IR OF CYCLOPENTADIENE
TABLE I-Continued Observed " cm-' 17 17 17 17 17 17 18 18 18 18 18 18 18 18 18 18 18 18 18 18 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 19 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20
2 3 2 1 0 1 6 7 5 6 4 5 3 4 2 3 1 2 0 1 16 17 15 12 13 10 11 9 10 7 8 6 7 5 6 4 5 3 4 2 3 1 2 0 1 11 12 10 11 8 9 5 6 4 5 3 4 2 3 1 2
15 15 16 16 17 17 12 12 13 13 14 14 15 15 16 16 17 17 18 18 3 2 4 1
7 9 9 10 10 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 9 9 10 10 12 12 15 15 16 16 17 17 18 18 19 19
18 18 18 18 18 18 19 19 19 19 19 19 19 19 19 19 19 19 19 19 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21 21
3 4 3 2 1 2 7 8 6 7 5 6 4 5 3 4 2 3 1 2 17 18 16 13 14 11 12 10 11 8 9 7 8 6 7 5 6 4 5 3 4 2 3 1 2 12 13 11 12 9 10 6 7 5 6 4 5 3 4 2 3
15 15 16 16 17 17 12 12 13 13 14 14 15 15 16 16 17 17 18 18 3 2 4 7 7 9 9 10 10 12 12 13 13 14 14 15 15 16 16 17 :7 18 18 19 19 9 9 10 10 12 12 15 15 16 16 17 17 18 18 19 19
673.8007 673.8007 673.8161 673.8161 673.8322 673.8322 674.3010 674.3010 674.3130 674.3130 674.3261 674.3261 674.3396 674.3396 674.3553 674.3553 674.3707 674.3707 674.3878 674.3878 674.7427 674.7527 674.7608 674.7900 674.7900 674.8071 674.8071 674.8165 674.8165 674.8380 674.8380 674.8499 674.8499 674.8632 674.8632 674.8771 674.8771 674.8926 674.8926 674.9081 674.9081 674.9259 674.9259 674.9437 674.9437 675.3433 675.3433 675.3528 675.3528 675.3741 675.3741 675.4138 675.4138 675.4284 675.4284 675.4453 675.4453 675.4619 675.4619 675.4808 675.4808
ObscalC Xl@V -7 -7 -2 -2 -1 -1 4 4 3 3 3 3 -2 -2 5 5 0 0 2 2 -3 -7 2 -2 -2 2 2 1 1 -1 -1 -3 -3 -1 -1 -3 -3 3 3 -2 -2 7 7 7 7 -1 -1 -1 -1 -5 -5 -2 -2 -6 -6 4 4 1 1 10 10
_~
L-n-'
Observed N 20 20 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 26 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30 30
20 20 4 3 5 8 8 10 10 12 12 13 13 15 15 16 16 17 17 4 5 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29
obsxalc
N
Y
K,
; ",,-I
xl@;
21 21 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 27 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31 31
1 2 23 24 22 19 20 17 18 15 16 14 15 12 13 11 12 10 11 27 26 20 21 19 20 18 19 17 18 17 17 15 16 14 15 13 14 12 13 11 12 10 11 9 10 8 9 7 8 6 7 5 6 4 5 3 4 2 3
20 20 4 3 5 8 8 10 10 12 12 13 13 15 15 16 16 17 17 4 5 11 11 12 12 13 13 14 14 15 15 16 16 17 17 18 18 19 19 20 20 21 21 22 22 23 23 24 24 25 25 26 26 27 27 28 28 29 29
675.4994 675.4994 678.4842 678.4899 678.5017 678.5336 678.5336 678.5529 678.5529 678.5750 678.5750 678.5871 678.5871 678.6145 678.6145 678.6296 678.6296 678.6455 678.6455 680.6046 680.6107 680.6782 680.6782 680.6896 680.6896 680.7025 680.7025 680.7158 680.7158 680.7292 680.7292 680.7447 680.7447 680.7614 680.7614 680.7775 680.7775 680.7959 680.7959 680.8146 680.8146 680.8347 680.8347 680.8549 680.8549 680.8771 680.8771 680.8992 680.8992 680.9232 680.9232 680.9480 680.9480 680.9744 680.9744 681.0009 681.0009 681.0289 681.0289
9 9 -2 -3 2 0 0 3 3 3 3 1 1 1 1 2 2 0 0 3 -5 0 0 -1 -1 4 4 4 4 -4 -4 0 0 7 7 -2 -2 3 3 1 1 3 3 -3 -3 1 1 -6 -6 -4 -4 -3 -3 4 4 2 2 5 5
~msErr.x
0.000383
cm-'
on-'
included in the fit but later simulations were able to reproduce the merged lines with high accuracy. The more complicated patterns at higher N were assigned with the help of preliminary simulations including relevant nuclear spin statistics for intensities. Assuming a planar carbon-ring structure, CPD contains three pairs of protons in which each proton interchanges with its partner under the C2 operation of the Czv
108
BOARDMAN ET AL.
point group. Since for the exchange of an odd number of fermion pairs, the total wavefunction must be antisymmetric, each rotational level is given a weight thus:
e
e
0
0
e
0
0
e
Symmetry species in C,
Weight
A A B B
7 7 9 9
This manifests itself most obviously in the Kc = 1 (outer) components of the low-N P- and R-branch transitions. Generally, in R (N = even) transitions the lowerwavenumber Kc component (with the lower of the two possible K, values) has lower intensity than the higher-wavenumber component. In R (N = odd) transitions the intensity ratio reverses. For P (N = even) transitions the lower-wavenumber Kc = 1 component has the greater intensity. These effects may be seen in Figs. 2 and 3. In a final fit of the 420 P- and R-branch lines given in Table I, the ground-state parameters were fixed to the microwave spectrum values (5). The upper state parameters resulting from the least-squares fit are given in Table II and the %,bs_calc in Table I. Although the Q-branch features were not included in the fit, they were used to test the final parameters by simulating the room temperature and jet spectra. The Qbranch lines shown in Fig. 1 are blue shaded and both the regular structure on the
TABLE II Molecular Parameters (cm-‘) for the Ground-State and 26, Levels of Cyclopentadiene ( 1c Uncertainties Given in Parentheses) Ground State.(a)
Level 26,
A
0.28106478
(7)
0.2804694
(11)
B
0.27437782
(7)
0.2740983
(10)
C
0.14247981(l)
0.1425197
(3)
108,
AN
5.15(l)
5.178 (62)
108,
AM
-2.04(4)
-1.80 (25)
108,
AK
5.61(3)
5.09 (35)
log,6,
2.04 (5)
2.083(33)
108,
2.955(9)
2.81(10)
6,
Go (26;)
663.84800(5)
(a) fixed at the values from the mm wave spectrum (ref. 5)
HIGH-RESOLUTION
IR OF CYCLOPENTADIENE
109
high-wavenumber side and irregular lines on the low-wavenumber side in Fig. la (not visible in the lower S:N ratio FTIR spectra) are definite spectroscopic features due to high rotational transitions which are suppressed in the colder jet spectra. The appropriate simulations with assumed Doppler profiles of 0.002 cm-’ FWHM are shown in Figs. lc and d. Representative simulations of the P and R branches are shown in Figs. 2 and 3 (c and d). One further feature of the room temperature FTIR spectrum should be noted: a closely similar, but weaker Q branch was detected between 664.1 and 664.2 cm-‘. DISCUSSION
The results of the present study not only consolidate the earlier assignment of this band to a vibration of B2 species but also confirm the value of 664 cm-’ for the band center. The position of the band origin predicted from the fit, Go= 663.848 cm-‘, is in excellent agreement with the narrow, intense feature at 663.847 cm-’ in the coldest jet spectrum (Fig. 1b) . This linelike feature is an accumulation of lines with K, = N. At low temperatures only a few low-N lines contribute to its intensity but at higher temperatures many higher N levels are populated, giving rise to the blue shading in Fig. 1a. Lower-wavenumber Q-branch features correspond to aggregations of transitions with K, = N - 1, Kc = N - 2, etc. The Q-branch feature, resembling the fundamental at 664.2 cm-‘, can be tentatively assigned to the 26627 f hot band. The simulation of the Q branch reproduces not only the intensity and shadings of the main features but also the detailed fine structure in the room temperature spectrum. This includes the broad rising background extending to lower wavenumber, which disappears in the cold spectrum. Simulations of the lowest-temperature spectra (Fig. 1d) showed that although their rotational distributions were not completely thermalized, they corresponded to T,, N 25 K. In the P and R branches, the Kc patterns of each P and R transition are relatively insensitive to temperature. In addition, only one P or R transition appears on a single laser mode. A quantitative derivation of T,, from them is therefore not readily achieved. In conclusion, the planarity of the carbon ring is confirmed by the present study. There are no significant changes in the inertial defect or centrifugal distortion constants on excitation of this mode, which might have been expected from comparison with other planar molecules manifesting nonrigidity. This mode of CPD seems free of any obvious perturbations and the successful analysis of its IR spectrum supports the symmetry assigned to this mode in both the low-resolution vibrational and force-field analyses. ACKNOWLEDGMENTS. We thank the SERC for a studentship for S.R.B. and Unilever Research, Port Sunlight studentship for S.A.B. in addition to several equipment grants. We also thank Professor the loan of equipment. REFERENCES 1. A. C. LEGON, Chem. Rev. 80,231-262 (1980). 2. V. W. LAURIE, J. Chem. Phys. 24635-636 (1956).
Laboratory, for a B. A. Thrush for
110
BOARDMAN
ET AL.
3. L. H. S~HARPENAND V. W. LAURIE,J. Chem. Phys. 43,2X5-2166 ( 1965). 4. D. DAMIANI, L. FENETTI,AND E. GALLINELLA,Chem. Phys. Lett. 37,265-269 (1976). 5. M. BOGEY, C. DEMUYNCK,AND J. L. DESTOMBES, J. Mol. Spectrosc. 132,277-279 ( 1988). 6. E. GALLINELLA,B. FORTUNATO,AND P. MIRONE,J. Mol. Spectrosc. 24,345-362 ( 1967). 7. K. B. THAKUR, V. A. JOB, AND V. B. KARTHA,J. Mol. Spectrosc. 112,340-346 ( 1985). 8. G. MAGNCJSSON, J. Org. Chem. SO, 1998 (1985). 9. P. B. DAVIES,N. A. MARTIN, AND M. D. NUNES, Spectrochim. Acta A 45,293-298 ( 1989). IO. G. GUEIXHVILI AND K. NARAHATIRAO, “Handbook of Infrared Standards,” Academic Press, Orlando, FLy 1986. 11. E. CASTELLUCI,P. MANZELLI,B. FORTUNATO,E. GALLINELLA,AND P. MIRONE, Spectrochim. Acta A 31,451-461 (1975). 12. J. K. G. WATSON, in “Vibrational Spectra and Structure” (J. R. Durig, Ed.), Vol. 6, pp. 2-89, Elsevier, Amsterdam, 1977.