Infrared kinetic spectroscopy of C2H and C2D

Journal of MoZecuZarStructure, 190 (1988) 41-60 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

41

INFRARED KINETIC SPECTROSCOPY OF C2H AND CzD*

J.W. STEPHENS, Department (U.S.A.)

WEN-BIN YAN, MARILYN L. RICHNOW, H. SOLKA** and R.F. CURL

of Chemistry and Rice Quantum Institute,

Rice University,

Houston, TX 77251

(Received 6 January 1988)

ABSTRACT Color center laser kinetic spectroscopy has been used to investigate the spectrum of C2H and CzD. For CzD four ‘Z++- ‘I+ “hot” bands originating from the (O,O,l) k ‘C’ level and one new 2II+2X’ band originating from the ground vibronic level have been observed in the region 3350 to 3750 cm-’ and are analyzed. The focus of interest for C&His the observation and assignment of the CH stretching fundamental. In this search, three ‘C’t’Z+ bands originating from the ground vibronic level have been observed in the region 3250 to 3600 cm-’ and are analyzed. However, with 13Csubstitution to 13C13CH,no bands were found with an isotopic shift from the three “C&H bands as small as is expected (ca. 20 cm-‘) for the CH stretch.

INTRODUCTION

The spectrum of C2H has been studied extensively by ESR [1,2], matrix isolation IR spectroscopy [3,6], LMR [7], microwave spectroscopy [ 8-101, color center laser spectroscopy [ 11-151, and diode laser spectroscopy [ 16,171. As a result of these investigations, the ground state structure, the bending ( v,) and CC stretching ( v3) fundamental frequencies, and the lowest lying electronic transition, A211+-%2C+, are known. Of great current interest are the location of the CH stretch fundamental (vi) and the energy relationship between the A and 2 potential surfaces and the development of an understanding of the vibronic coupling between a and 2. The development of an understanding of the spectroscopy of C,H has been greatly aided by ab initio calculations [l&22]. Figure 1 shows the qualitative nature of the expected potential surfaces. Figure 1 (a) reminds us that the A”II surface is expected to split as the result of the Renner-Teller effect into two components, one of symmetry A’ and the other of symmetry A” as the molecule bends away from the linear geometry. Figure 1 (b) depicts the rela*Dedicated to the memory of Professor Walter Gordy. **Institut fiir Angewandte Physik, Universitit Bonn, Wegelerstrasse 8, D 5300 Bonn 1, Federal Republic of Germany.

0022-2860/88/$03.50

0 1988 Elsevier Science Publishers B.V.

42

5 CC Bond

Length

(A)

Fig. 1. (a) Qualitative potential surface of the 2 and A states of C*H as a function of bending angle. The upper surface is being split by the tinner-Teller effect. (b) Ab initio potential surfaces of C.$H as a function of C-C distance [23] at the linear geometry showing the crossing of the 2 and A states. For a somewhat nonlinear geometry the A state would be split into A’ and A Wsurfaces and the crossing with the A’ 2 state would be an avoided one.

tionship between the two surfaces in the linear geometry predicted [23] by most recent ab initio calculation [ 221 as a function of CC distance. In looking at Fig. 1 (b) , one must keep in mind that as the molecule becomes non-linear the a upper potential curve will split into two curves with the lower having A” symmetry and the upper having A’ symmetry and that the X potential curve of symmetry A’ can cross the A” state, but there will be an avoided crossing with the upper A’ surface. Thus there is a cusp crossing between the X state and one of the d state surfaces. As this crossing is expected to occur near the e_quilibriumgeometry of the A state, severe mixing of the lowest levels of the A state and excited vibrational levels of the X state is expected, as these have the same symmetry and are of about the same energy. Except for 2C- levels which are of A ” symmetry, A state levels do not belong strictly to either the A’ or the A” surfaces because of orbit-rotation interaction mixing as the system vibrates near the linear geometry. Severe vibronic mixing through a cusp crossing is known for NO2 [ 241. However, in that case, the interaction has not been analyzed in detail because the cusp crossing occurs at fairly high energies and the density of ground state excited vibrational levels in high near the cusp crossing. On the other hand, the cusp crossing in C2H is at a low energy, and the density of ground state excited vibrational levels is consequently low. Thugsthere-is hope, in the long term at least, that the vibronic coupling between A and X states of C2H may be analyzed. If the vibronic coupling in C,H is to be analyzed, then as much information as possible about the energy levels of the molecule must be obtained. In pre-

43

vious papers [11-E], a number of 211t 2C + bands involving the ground vibronic state as the lower level (five for C2H and two for C2D) and a number of “hot” bands have been reported. The “hot” bands are of two types. One band, involving a transition of the first excited state of the bending mode, X(0,1,0), to a 2C- level of the A state thought to belong to A (O,l,O), is observable with room temperature thermal populations and has been observed for C,H and C,D. A second set of three bands of C2H all of type ‘C+ t2C’ and all originating from the first absorption IR kinetic spectroscopy with the population of the lower levels brought about by the excess energy in the 193 nm flash photolysis of acetylene. In this paper, we continue reporting the spectrum of C2H (and its isotopomers) obtained by color center laser spectroscopy. The method used for all the results reported in this paper is IR kinetic spectroscopy with 193 nm excimer laser flash photolysis of acetylene isotopomers and color center laser probing. Here the observation and analysis of an additional band of C,D of type 211+-2C arising from the ground state and four 2C + t2C + bands of C2D originating from the (O,O,l) state populated by the photolysis are reported. In addition, our efforts to observe and assign the CH stretching fundamental which resulted in the observation and analysis of three C,H bands of type 2C++2C’ all involving the ground state are described. EXPERIMENTAL

For these observations, as for those made previously using flash photolysis were obtained by observing the transient absorption of C,H about 0.6 ,us after the flash photolysis of acetylene with an ArF excimer laser (193 nm). The apparatus used has been described previously [ 141 except that during the course of these measurements the single pass flash photolysis cell was replaced by a multiple reflection (“White”) cell. In this arrangement, the excimer beam is introduced just below the “D” mirrors and travels diagonally upwards through the infrared beam until it is intercepted by a beam block placed just above and in front of the upper row of infrared beam spots on the notched mirror. This arrangement provides greatly enhanced sensitivity in comparison with the single pass absorption arrangement used previously. The signal-to-noise ratio is increased by a factor of ca. 15. Several gas mixture and flow conditions were used in these observations. In the initial measurements about 0.35 torr of acetylene in about 20 torr of helium was flowed slowly through the cell so that the sample was exposed to ca. 40 excimer shots. For the purpose of observing transitions from the vibronic ground state, Kanamori et al. found [ 161 that more rapid vibrational relaxation was obtained by replacing helium with hydrogen as the carrier gas, and in our experiments up to 25 torr of H2 was used for this purpose. When H13C13CH was [ 141, the spectra

44

used, the measurements were carried out in a static system with about 20 torr of H2 and 0.4 torr of 13C-acetylene. The volume irradiated is about 100 cm3 compared with a cell volume of about 17 1, resulting in a working time of 20 min at 20 pulses s-l before the cell requires a new fill of acetylene. In order to minimize absorption of the excimer beam before it reaches the region probed by the IR, i.e. as it passes through the region below that probed, the box containing the “D” mirrors was gently flushed with He or Hz. This also minimized the rate of appearance of an ultraviolet-absorbing solid deposit on the input window. In the case of the static fills, introduction of first the 13Cacetylene and then Hz from the input window end was found sufficient to reduce the formation of the solid deposit to a level which could be tolerated. Apparently this method of filling pushed the 13C-acetylene to the opposite end of the cell from the input window, yet left enough in the photolysis path to produce adequate signals. The time for diffusion from the opposite end of the cell to the input window was estimated to be comparable to observation time. For the normal isotope, commercial acetylene, although known to be heavily contaminated with acetone, was used without further purification. For C&D the &-acetylene was produced by reaction of D,O (99.8 atom % D) with calcium carbide (“Baker” grade). The 13C2-acetylene (90 atom % 13C) was purchased from MSD Isotopes. Normally the color-center laser was scanned in high-resolution mode; however, due to the high cost of 13C2H2, some of the 13C scans were performed in the faster “mode-hopping” mode where the cavity of the color-center laser remains fixed and only the etalon and grating are scanned. The high-resolution spectra were calibrated by simultaneously recording the spectrum of water (above 3500 cm-l) [25] or ammonia (below 3500 cm-l) [26] and the interference fringes of a temperature-compensated Invar reference cavity (FSR= 497 MHz). For the mode-hopping scans, these marker fringes are not resolved, so the spectra were calibrated by simple interpolation between reference lines. The precision is ca. 0.001 cm-’ for the high-resolution measurements and ca. 0.004 cm-l for the mode-hopping scans. (As is usually the case, the precision of peak measurement for mode hopping is better than the resolution. ) OBSERVATIONS

The spectrum of C2D was explored in the region from about 3330 to 3850 cm-‘. One new band of the type zII+2C+ involving the ground vibronic state and four bands of the type 2C + c ‘C’ all with a common lower level thought to be the first excited state of the CC stretch were observed, analyzed, and assigned. For the normal isotope, the aim was to use the improved sensitivity of the multiple reflection cell in a search for the as yet unobserved CH stretching

45

fundamental. The CH stretch is expected to be in the region 3600- 3250 cm-‘, and therefore this was the region explored. Three bands of the appropriate ‘C+ +2C+ symmetry involving the required ground vibronic state were observed, assigned, and analyzed. Of these three bands, two have B’ rotational constants of the magnitude expected for the desired (l,O,O) upper state. In order to further test whether either of these bands is the CH stretch, 13C was studied as the CH stretch is expected to have a relatively small frequency shift (ca. 20 cm-l) on isotopic substitution. Unfortunately, none of the 2C+2C bands was found to have the desired small isotope shift. One excited state and three ground state 13C bands were observed and analyzed, however. RESULTS

Ground state 211+2C’

band of C,D

In previous work [ 151 the magnetic rotation spectrum of a discharge in argon over a deposit of d-polyacetylene was recorded in the region 3920-3200 cm-l. Three bands of C,D with origins at 3856, 3729 and 3426 cm-’ were observed and analyzed. Two of these bands, 3856 and 3426 cm-‘, are absorptions from the ground vibronic state of type zII+2C+ while the third is of symmetry type 2C- t211 with the lower state thought to be the first excited state of the bending vibrations, (0,&O). A very strong band of CO, the 04-2 transition of the a’ 3Cta311 system, was also observed near 3534 cm -‘. When this region was reinvestigated using the flashphotolysis of d-acetylene, a new band of C,D at 3513 cm-l of type 211t2C involving the ground state was recognized. In the magnetic rotation spectrum, the lines of this band are overshadowed by the very strong lines of CO in the same spectral region. When the spectra are plotted with the CO lines on-scale, the C2D lines are so small that their lineshapes are not readily recognized. More careful examination, however, reveals the characteristic magnetic rotation lineshapes found previously for the 3856 and 3426 cm-’ bands. The observed lines of this band are listed in Table 1 and the resulting spectroscopic constants are given in Table 2. In the 211 Hamiltonian used to obtain the constants listed in Table 2, we have changed the signs in front of q, qD, p and pn to conform to the K-type doubling adopted by Brown [ 271. These terms now appear as +f[p+pnN2,S+N++S_N_]+-j(q+qn~2)(N:+N%)

(I)

rather than with the opposite signs as given in equation (2 ) of ref. 13. It should be noted that the constants given in ref. 15 also use our previous choice of sign. Brown’s choice of sign conforms to the /i-doubling convention but has the opposite sign than is usual for Z-type doubling.

46 TABLE 1 High-resolution line positions for the C&D3513 cm-’ band” N

R-branch 11

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

3512.173 3514.909 3517.402 3519.706 3521.853 3523.871 3525.763 3527.544 3529.217b 3530.792 3532.264 3533.642 3534.926 3536.110 3537.210 3538.217 3539.136 3539.975 3540.730 3541.412 3542.025

Q-branch

P-branch

22

11

22

3519.317 3520.950 3522.643 3524.341 3526.016 3527.647 3629.217b 3530.715 3532.142 3533.495 3534.765 3535.949 3537.054 3538.074 3539.008 3539.859 3540.621 3541.301 3541.893 3542.405 3542.834

3509.771b 3510.134 3510.270 3510.234 3510.061 3509.771b 3509.381 3508.894 3508.320 3507.661 3506.921 3506.103 3505.209b 3504.237 3503.192 3502.078 3500.895 3499.643 3498.329 3496.951 3495.515 3494.023 3492.597

3515.419 3514.551 3513.827 3513.180 3512.556 3511.930 3511.274 3510.57Sb 3509.832 3509.031 3508.169 3507.247 3506.262 3505.209b 3504.097b 3502.912 3501.665 3500.33Sb 3498.984 3497.553 3496.060 3494.511

11

22

3504.956

3510.57Sb 3507.284 3504.097b 3500.988 3497.877 3494.740 3491.560 3488.319 3485.013 3481.633 3478.178 3474.643 3471.028 3467.329 3463.547 3459.693 3455.735 3451.704 3447.589 3443.394

3502.871 3500.558 3498.051 3495.393 3492.597 3489.681 3486.654 3483.521 3480.288 3476.952 3473.524 3470.004 3466.389 3462.687 3458.893b 3455.016 3451.058 3447.023

“In cm-‘. bLines are overlapped. TABLE 2 Constants for the C,D 3513 cm-’ ‘%t’C+ 211upper state

band” ?X+ (000) lower state

Constant

Value

Constant

B Dx106 A Y P 9 q,x107 PDX lo5

1.16245( 1) 1.73(5) -6.363(2) 0.0020 (1) 0.0013(3) -0.00881(2) 8.6(9) -2.9(l)

B Dx106 Y

“In cm-‘. bFrom ref. 10.

ZD. Lines fitted

Value 1.20310b 2.256b -0.001864b 3513.477(l) 0.003 1041125

47 TABLE 3 High-resolution line positions for the C2D 3406 cm-’ band* N

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

P-branch

R-branch 11

22

11

22

3408.337 3410.675 3412.989 3415.275 3417.531 3419.755 3421.941 3424.090 3426.195 3428.255 3430.263 3452.218 3434.115 3435.951 3437.722 3439.426 3441.081 3442.655 3443.955 3445.466 3446.766 3448.013 3449.183 3450.165 3451.332

3410.675 3412.973 3415.256 3417.502 3419.718 3421.899 3424.042 3426.141 3428.195 3430.198 3432.146 3434.040 3435.869 3437.633 3439.332 3440.963 3442.549 3443.860 3445.327 3446.653 3447.891 3449.052 3450.049 3451.194

3403.600 3401.193 3398.771 3396.320 3393.842 3391.336 3388.798 3386.222 3383.611 3380.959 3378.262 3375.513 3372.714 3369.857 3366.942 3363.962 3360.918 3357.810 3354.657

3401.193 3398.755 3396.297 3393.814 3391.302 3388.757 3386.176 3383.558 3380.901 3378.195 3375.443 3372.637 3369.776 3366.855 3363.868 3360.819 3357.705 3354.544

3347.981 3344.554

3347.840 3344.431

3337.500

3337.465

“In cm-‘.

“C’+“C’X(O,O,l)

bands ofC,O

Previously [ 141 three bands of C,H arising from the first excited vibrational state of the CC stretch, (O,O,l) of type 2C + t2C + were observed in the flash photolysis of acetylene. These bands were interpreted as transitions to levels which are principally highly excited vibrational levels of the X state with the transition allowed by vibronic mixing as-a result of the difference in the permanent electric dipole moments of the X and A states. In the present study, four similar bands of C2D at 3406, 3464, 3605 and 3717 cm-’ were observed and assigned. Their transition frequencies are listed in Tables 3-6. As was found for the similar bands of C2H, the upper states of these bands are usually perturbed. Therefore, the lower state, which is unperturbed, was fitted simultaneously using lower state combination differences from all four bands. Then

46 TABLE 4 High-resolution line positions for the C&D3464 cm-’ band” N

0

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

R-branch

P-branch

11

22

11

3466.040 3468.451 3470.880 3473.334 3475.813 3478.318 3480.856 3483.436 3486.068 3488.760 3491.521 3494.368 3497.310 3500.360 3503.537 3506.851 3510.315 3513.934 3517.717 3521.664

3468.451 3470.880 3473.327 3475.803 3478.303 3480.840 3483.420 3486.049 3488.738 3491.500 3494.344 3497.287 3500.338b 3503.513 3506.832 3510.292 3513.914 3517.696 3521.646

3461.259 3458.893b 3456.543 3454.214 3451.904 3449.617 3447.361 3445.141 3442.960 3440.833 3438.767 3436.772 3434.863 3433.051 3431.350 3429.778 3428.348 3427.063 3425.936

22

3458.893b

3456.536 3454.204 3451.892 3449.601 3447.343 3445.119 3442.938 3440.809 3438.742 3436.744 3434.835 3433.023 3431.323 3429.754 3428.313 3427.033 3425.909

“In cm-‘. bLines are overlapped. TABLE 5 High-resolution line position for the C2D 3605 cm- ’ band* N

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

R-branch

P-branch

11

22

3607.783 3610.158 3612.535 3614.902 3617.274 3619.642 3621.996

3610.158 3612.535 3614.902 3617.274 3619.642 3621.996

b

b

3626.684 3629.004 3631.303 3633.570 3635.793 3637.954

3626.703 3629.033 3631.343 3633.625 3635.867 3638.049

“In cm-‘. bLines are overlapped.

11

22

3600.636 3598.252 3595.863 3593.473 3591.079 3588.684 3586.279 3583.871 3581.448 3579.011 3576.551 3574.066 3571.532 3568.942 3566.269

3600.636 3598.252 3595.863 3593.473 3591.079 3588.684 3586.279 3583.871 3581.463 3579.034 3576.588 3574.116 3571.603 3569.033 3566.359

49

TABLE 6 High-resolution line positions for the C,D 3717 cm-’ band” N

R-branch

P-branch 11

22

3726.454

3709.832

3709.819

3728.813

3707.439

3707.420

11

22

2 3 4 5 6 7 8 9 10

3724.105

3724.095

3726.476 3728.834 3731.195

3731.167

3705.039

3705.018

3733.533

3733.521

b

3702.614

3735.912

3735.874

3700.241

3700.207

3738.273

3738.228

3697.839

3697.800

3740.634 3742.999

3740.587 3742.946

3695.435

3595.395

11

3745.369

3745.311

3693.037 3690.640

3692.988 b

12

3747.750 3750.140

3688.249

3688.194

3685.865

3685.803

3683.489 3681.126

3683.422

3678.765 3676.437

3678.686

0 1

13 14 15

3753.530

3750.066 3752.451

3754.942

3754.852

16 17

3681.048 3676.340

“In cm-‘. bLines are overlapped.

TABLE 7 Constants for the C*D 21++-21+ bands” Upper state

3406 cm-’

3464 cm-’

3605 cm-’

3717 cm-’

band

band

band

band

1.17891(3) 2.26(2)

B Dx105

0.0036( 1) 3405.985 (1)

Y ZD. Lines fitted “In cm-‘.

Common lower state r%I+ (001) parameters

0.004 52194

%imuItaneous

1.19790(2) -4.108(6) -0.00036(9) 3463.647 ( 1) 0.004 69/78

1.19032(5) 1.39(4) -0.0022(Z) 3605.398(l) 0.003 36156

This work

1.18853(l) -0.432(6) 0.00259(7) 3716.983(l) 0.002 51/55

least squares fit of the lower state combination

1.19093(l) 0.253 (3) -0.0022(2)

and Hirota [28] Kanamori

1.1909327(29) 0.25469(65) -0.002441(15)

0.002 120/124b differences of the four bands

50

those transitions involving levels of the upper state which were felt to be least perturbed were fitted with the lower state constants fixed. The resulting constants are given in Table 7. The lower (O,O,l) state has been observed and analyzed by diode laser kinetic spectroscopy by Kanamori and Hirota [ 281, and our constants are compared with their more accurate ones in Table 7. The search for the x CH stretch: three ‘C t”C+&O,O,O)

bands of C2H

Two of the three fundamental vibrational frequencies of C2H, the CC stretch ( v,) and the bend ( v,), are now accurately known from high resolution studies. However, the CH stretching fundamental ( vl) is not yet assigned. Jacox

[4] proposed a band at 3611 cm-’ in C,H and a band at 2798 cm-l in C&D observed in matrix isolation as the CH and CD stretching fundamentals respectively. In general, there has been good agreement in frequency and intensity between the bands observed by matrix isolation and those observed in the gas phase in the color center region with the matrix band frequencies usually about lo-30 cm-’ above the gas phase origins. Under high resolution, the strong band observed at 3600 cm-’ in the gas phase [15] proved to be a ‘lIc2C+ band, the wrong symmetry type. We feel that the strong band observed in the matrix at 3611 cm-l undoubtedly corresponds to the gas phase 3600 cm-’ band. This, of course, does not preclude there being a weaker band near 3600 cm-’ corresponding to the CH stretch. (The region around 2800 cm-’ has not yet been studied in the gas phase). There are two other bands observed in the matrix isolation spectrum at 3555 and 3380 cm-l and assigned as arising from C2H [5] which we felt might possibly be the CH stretch and therefore deserving of investigation. Both of these bands, especially 3555 cm-‘, are considerably weaker than the 3611cm-’ band in the matrix isolation spectrum, and, indeed, our efforts to observe them by flash photolysis were initially unsuccessful. However with the signal-tonoise improvement obtained with the White cell arrangement, no difficulty was found in observing and analyzing these bands, which were found to have gas phase origins of 3547 and 3366 cm-‘. An S/N of 25 was obtained for the stronger lines of the weaker 3547 cm-’ band. Additional searching to lower frequency into the region where C4Hz absorption interferes in matrix isolation revealed a third even weaker band with its origin at 3299 cm-‘. Allthreeofthesebandsareoftype2C+t2C+ with the lower level the ground vibronic state, and were therefore possible candidates for the CH stretch. The transition frequencies are reported in Table 8 and the resulting rotation constants and band origins are reported in Table 9. In fitting these spectra, the ground state constants were fixed at the microwave values [ 81. With the exception of a few lines of the 3299 cm-l band which were not fitted, the spinrotation splitting was usually not resolved for these bands, so that the difference in the spin-rotation interaction constant, y, between the upper state and

51 TABLE 8 High-resolution line positions for the C&H*X + +*I + bands” N

3299 cm-’ band

3366 cm-’ band

3547 cm-’ band

R-branch

R-branch

R-branch

P-branch

0

3369.233

1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16

3372.058 3374.839 3377.577

17 18

3307.248 3310.248 3315.790 3318.518 3321.222 3323.889 3326.544 3329.162 3331.755 3334.317 3336.849 3339.352

3293.001 3290.034 3287.044 3284.028 3280.986 3277.918 3271.707 3268.562 3265.392 3262.194 3258.969 3255.721

3382.931 3385.550 3388.135 3390.687 3393.208 3395.701 3398.169

b

3344.271” 3344.255” 3346.677” 3346.660” 3349.050” 3349.033’

19 20 21 22 23 24 25 26

P-branch

P-branch

P-branch satellite

3549.640

3363.451 3360.492 3357.486 3354.445 3351.354 3348.241 3345.060 3341.858 3338.608 3335.358 3332.055 3328.737 3325.388 3322.008 3318.635 3315.228

3552.504 3558.150 3560.926 3563.663 3566.358 3571.601 3574.139 3576.613 3579.020 3581.353 3583.606 3585.787

3543.836 3540.897 3537.937 3534.947 3531.931 3528.881 3525.793 3522.665 3519.491 3516.266 3512.981 3509.643 3506.236 3502.755 3499.198 3495.572

3311.812

3491.863

3492.014

3308.365

3488.070

3488.155

3484.192 3480.226 3476.171 3472.028 3467.793 3463.468 3459.049

3484.268 3480.302 3476.250 3472.112 3467.885 3463.568 3459.161

3301.466 3294.508 3287.489 3283.956 3280.403

“In cm-‘. bLines are overlapped. ‘The two entries are for the two spin components. TABLE 9 Constants for the C,H ‘X++-‘C+

bands”

Upper state 3299 cm-’ band B Dx105 &. Lines fitted

1.44394(2)

0.460(g) 3298.853 0.002 (1) 24130

“In cm-‘. ‘From ref. 8.

3366 cm-’ band

3547 cm-’ band

1.43456(3) -1.07(2) 3366.363 0.002 (1)

1.44482(3) 3.83(4) 3546.750( 0.001 1)

17/30

15/38

Common lower state 92x+ (000) parameters 1.45683b 0.351b

0

2

4

6

6

IO

12

14

16

16

20

22

24

N’

Fig. 2. Residuals for the 3547 cm-’ C&H band showing the large deviations satellite lines (0) which appear suddenly at N’ = 16.

at large N’. Note the

TABLE 10 Line positions N

for the i3C,H 3290 cm-’ band” R-branch High-resolution

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 “In cm-’

P-branch Mode-hop

High-resolution

Mode-hop

3293.015 3295.660

3295.660

3298.293 3300.882 3306.002 3308.528 3311.010 3313.470 3318.294 3320.659 3322.999 3325.294 3327.560 3329.790

3284.901 3282.145 3279.364 3276.555 3273.722 3210.861 3265.065 3262.122

3287.627 3284.898 3282.141 3279.366 3267.559 3273.722 3270.864 3267.984 3265.065 3262.113 3259.156 3256.157 3253.122 3250.059 3246.986 3240.674

53 TABLE 11 Line positions for the i3C2H 3303 cm-’ band” N

High-resolution 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

P-branch

R-branch

3310.762

3321.052 3323.556

3333.317 3335.683

3345.005

Mode-hop 3305.443 3308.125 3310.761 3313.374 3315.964 3318.521 3321.050 3323.554 3326.033 3328.479 3330.913 3333.317 3335.691 3338.053 3340.398 3342.709 3345.011

High-resolution

Mode-hop

3299.996

3297.231 3294.436 3291.606

3297.230 3294.431 3291.603

3285.871 3282.958 3280.020 3277.055 3274.069 3271.054 3268.018 3264.958 3261.882

3285.876 3282.957 3280.021 3277.052 3274.065 3271.048 3268.020 3264.960 3261.874 3258.783 3255.665 3252.526 3249.371 3246.201 3243.012 3239.811 3236.590 3233.355 3230.108 3226.835 3223.551 3220.248

“In cm-‘.

the ground state is not determined. An apparent splitting was observed for some of the higher N levels of the 3547 cm-l band. However, in this case a satellite displaced from the main line to higher frequency by 0.15 cm-’ suddenly appears at N’ = 16in the P-branch and continues to the highest N’ value observed (N’ = 24) with the splitting initially dropping rapidly to about 0.07 cm-l and then rising gradually to 0.12 cm-‘. The 3547 cm-’ band is also unusual in that it cannot be fitted at high N with the normal rotational Hamiltonian. If the lower N levels are fitted with B' and D’, the calculated frequencies are far too small at higher N. This behavior is illustrated in Fig. 2 and suggests strongly that the upper levels are

54 TABLE 12 Line positions for the 13C,H 3464 cm-’ band” N

R-branch Highresolution

0

3466.429

1

3469.050

2 3 4 5 6 7 8 9 10 11 12 13

3471.609 3474.110 3476.551 3481.254

3487.848

P-branch Mode-hop

High-resolution

Mode-hop

3469.051 3471.596 3474.104 3476.555 3478.929 3481.252 3483.514 3485.712 3487.837 3489.916 3491.926 3493.880

3461.017 3458.222

b

3452.458 3446.459

3443.371 3440.220 3437.016 3433.746

b

3455.370 3452.460 3449.489 3446.455 3443.365 3440.223 3437.016 3433.742 3430.414 3427.027 3423.573

“In cm-‘. bLines are overlapped and not resolved in mode hopping.

being overtaken from below by some perturbing level. As B’ is already large in comparison with those values observed for other bands in this frequency region, this apparent overtaking by a state with an even higher B is rather surprising. The other two bands can be fitted within experimental error by the simple model with two parameters, B’ and D’. Kraemer et al. [22] have predicted a,=0.006 cm-l from the ab initio potential surface implying B = 1.451 cm-’ for the (l,O,O) level. Even if this estimate is poor, it seems to rule out the 3366 cm-’ band where the observed B’ would imply czl = 0.022. However, if the 3547 cm-’ band were assigned as v~, cyl would be 0.012 cm-l and correspondingly for the 3299 cm-’ band, cyl would be 0.013 cm-l. These possible a1 values seem more reasonable especially if one keeps in mind that the ab initio prediction [22] of cy3 is 0.010 and of cy2 is - 0.001 cm-’ while the observed a3 is 0.018 [ 161 and the observed cy2is 0.005 cm-’ [ 131. Therefore, in order to check whether either the 3299 or the 3547 cm -’ band was a viable candidate for the CH stretch, we investigated the spectrum of 13C2Hin the expectation that the 13Cshifts of these bands would reveal which, if either, of the 3547 or 3299 cm-l bands might be the better candidate for the CH stretch. 13C spectra When the 13Cspectrum was investigated in the regions 3186-3345 and 34203569 cm-l, four bands with origins at 3290, 3303, 3464 and 3546 cm-’ were

55 TABLE 13 Mode-hopping line positions for the 13C2H3546 cm-’ band” N

11 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

b

3546.632 3549.441 3552.023 3554.384 3556.567 3558.552 3560.398 3562.089 3563.635 3565.023 3566.269 3567.376 3568.345 3569.097

P-branch

Q-branch

R-branch

22 3551.100" 3552.754 3554.511 3556.020 3557.935 3559.669 3561.336 3562.902 3564.365 3565.711 3566.945 3568.062 3569.051

11

22

11

22

3540.789

b

3541.309 3541.196 3540.885 3540.408 3539.773 3538.984 3538.058 3536.997 3535.799 3534.474 b

3531.414 3529.677 3527.826 3525.829 3523.690 3521.419 3519.005 3516.454 3513.793

3547.284 3546.317 3545.405 3544.499 3543.562 3542.577 3541.502 3540.351 3539.100 3537.745 3536.282 3534.707 b

3531.203 3529.246 3527.190 3524.994 3522.662 3520.202 3517.587 3514.833 3511.954 3508.906 3505.714 3502.368 3498.867

3535.316 b

3530.299 3527.402 3524.290 3520.996 3517.525 3513.891 3510.126 3506.203 3502.127 3497.915 3493.564 3489.073 3484.379 3479.605 3474.672 3469.597 3464.380 3459.018 3453.485 3447.814 3441.981

3524.113 3520.304 3516.399 3512.395 3508.276 3504.050 3499.702 3495.233 3490.626 3485.900 3481.034 3476.030 3470.891 3465.619 3460.196 3454.634 3448.926

“In cm-‘. bLines are overlapped. “This line should be labelled RQ21(0).

observed and analyzed. The three lowest frequency bands are of type 2C + c2C + while the band at 3546 cm-l is of type 2n+-2C’. The three highest frequency bands (two 2C++2C’ and one 211t 2C’) have the same lower state. The B” value obtained by simultaneous fitting of the ground state combination differences of all three bands is 1.3681 cm-‘. This value is in excellent agreement with the value of 1.368 cm-l predicted by scaling the ground state “C B value (1.45683 cm-’ [9] ) by the ratio (13,13)B000/(12,12)B000 from the ab initio calculation [ 22 1. The band at 3290 cm-l originates from an excited vibrational state as evidenced by its rapid quenching by H2 and its smaller B” value. The observed transition frequencies and rotational constants of these bands are reported in Tables 10-15. On the basis of its similar rotational and spin-orbit constant and reasonable

56 TABLE 14 Constants

for the i3C,H ‘1’ c2,Y.’ bands” Upper state

B Dx106 & Lines fitted

Lower state

3290 cm-’ bandb

3303 cm-’ band

3464 cm-’ band

%2x+ (OOl)b,f

1.33886( 1) 6.2 (2) 3290.332 ( 1) 1.07” 39/39d

1.35302(2) -2.0(l) 3302.733( 1) 1.02’ 36/61d

1.33884(2) 6.0(2) 3463.753 ( 1) 1.09” 38/38d

1.35173(7) 2.8(2)

PC’

(0oo)e

1.36810(2) 3.1(l) 0.91” 48/50dF”

“In cm-‘. bConstants are from simultaneous fitting on upper and lower states. “Calculations are performed using weightings of 0.001 cm-’ for high-resolution lines and 0.004 cm-’ for modehopping lines: therefore standard deviations are normalized to unity. dTotal of both high-resolution and mode-hopping lines. ‘Calculated from least-squares fit on 3303 cm-‘, 3464 cm-‘, and 3546 cm-’ band combination-differences using both high-resolution and mode-hopping lines. fLower state of the 3290 cm-’ band. TABLE 15 Constants

for the 13C2H 3546 cm-’ 211+-2x+ band”

211upper stateb B DxlO” A 4: 4 q,x106 pJJx106 &l. Lines fitted

1.30851(3) 8.5(l) -8.423(3) - 0.0086 ( 1) -0.0032(5) -0.00717(3) 2.7(2) 9(4) 3545.567 (1) 0.003 60/111

“In cm-‘. bSee Table 14 for lower state constants.

13Cshift, the 3546 cm-’ band has been assigned as the 13Canalogue of the 3600 cm-’ band. Likewise, the B andD constants for the 3290 cm-’ 13C,H 2C++2C+ hot band are in excellent agreement with the values predicted by scaling the parameters of the 3320 cm-’ “C2 2C’t2C’ hot band in a similar manner. There remain two ‘C’ +2 C + ground state bands with origins at 3303 and 3464 cm-‘. The 13Cshift and similarity of B’ -B” of the 3303 cm-’ band to that of the 3366 cm-’ “C ‘C’t2C’ suggests that they are analogous bands. The 13Cshift is reasonable for the 3464 cm-’ band to be the analogue of the

57

2C+t2C+ 3547 cm-l ground state band. However, B’ -B” (=0.03 cm-‘) for the 3464 cm-’ band is much larger than B’ -B” ( = 0.012 cm-‘) for the 3547 cm-’ band, putting in doubt whether these two bands are analogues. No possible 13C analogue was found for the 3299 cm-’ band. The ab initio calculation predicts the CH stretch at 3497 cm-l with an isotope shift of 15 cm-l upon double 13C substitution. The 13C isotope shift may also be calculated from the product rule and the matrix isolation 12C12CH and 13C13CH CC stretch frequencies (1846.2 and 1786.1 cm-’ [5] respectively). Such’s calculation predicts that the ratio of (13,13)vIto (12,12)v1 is 0.99141 which gives a shift of about 28 cm-’ for the assignment of z+ to 3299 cm-l and about 30 cm-’ for the assignment of vI to 3547 cm-‘. Clearly we have not found a 13C shift similar to this for either 13C ground state 2C++2C’ band observed. DISCUSSION

Vibronic mixing and electronic origin The additional 211t2C’ band of C2D at 3513 cm-l reported in this work substantially alters the picture previously given [ 151 of the vibronic mixing in C2D and changes the estimated location of the vibronic origin of C,D. The upper levels of the 21YI e2C+ bands found in the color center region have spinorbit interaction and rotational constants intermediate between those expected for the A state and those expected for the X state. As was described in previous work [ 151, an estimate of the mixing between the X and A states for a particular 211state can be made from comparison of either the observed value of A or the observed_value of B with the expected values of the corresponding quantities for pure A and pure X states. Figure 3 depicts the results obtained for all upper state levels including the new 3513 cm-’ band of C2D and should be compared with Fig. 3 of ref. 15. With the inclusion of the new C2D band, the sum of fractionalA state character over all bands is close to unity for both C2H and C2D as would be expected if these bands arise from sharing a single A”lI level, the vibronic origin (O,O,O), with several 211 states arising from excited vibrational levels of X. Thus the sum of the A values for C,H is -20.56 cm-’ and for C2D is - 19.78 cm-‘, and we assumed [ 151 by analogy to other similar molecules that a reasonable value of A for the 2 state of C2H might be - 25 cm-‘. Likewise the sum of the fractional A characters estimated from B is 1.37 for C&H and 1.16 for C2D. It is not surprising that these estimates add up to more than 1, since they are based on the (O,O,O) B for a pure X state and a3 is 0.018 cm-’ [ 161. Thus a ground state vibrational level with one quanta excitation of v3 would appear to have 17% A character. The vibronic origin of each isotopic species may be estimated by averaging the origins of the 211+ ‘C’ bands weighted by their A values

58

CCD

CCH 3600

3693

3786

4012

4108

E

BR

3426

3513

3856

I

--

Fig. 3. The spin-orbit interaction constants A and the rotational constants B of the ‘TI levels of C2H and C&D in comparison with the expected values (indicated by t_he horizontal lines) for pure A and pure X states. The A constant is expected to be zero for the X state and about - 25 cm-’ for the A state. The B values of the ground vibronic state were used for the B constants for the pure X states of the two isotopes and the B values of the 3772 and 3729 cm-’ upper states were used for the I3 values of the pure A states of C&H and C,D respectively.

CAivi ~(W,O) =*

(2)

This calculation gives 3772 cm -’ for the C,H origin and 3697 cm-’ for the C&Dorigin. It should be noted that the A (0,1,0)2C-+~(0,1,0)211 origin for C,H is 3772 cm-’ while the analogous origin in C&Dis 3729 cm-l. These numbers imply, somewhat surprisingly as v2’ is expected to be greater than v~“, that v2’ (~-E)‘/~z:~“. They also indicate that the isotopic shift in the zero point energy upon deuterium substitution of the A state is larger than that of the ground state. Assignment of the 2C’+2C’

A(O,O,l) C2D bands

As has been discussed previously in connection with similar bands of C2H [ 141, the 2C+ c2C + %( O,O,l) bands of C&Dare transitions to level which are

primarily excited vibrational states of the 2 electronic state and obtain their electric dipole transition moment from vibronic coupling between the A and .% state [ 291. In the C,H case, because of the uncertainties in the value of Y, and in the anharmonicity constants, only very tentative assignments could be proposed for the observed 2C++2 C’ bands. For C2D this problem is aggravated because the energy range is the same as for C2H, the vibrational frequencies are all lower, and therefore the density of vibrational levels is higher. When transition frequencies are predicted for C,D in the region by extrapolation of the ab initio calculations 122,231,ten 2C++2C+ bands originating from (O,O,l) are expected in the region 3200-3800 cm-‘. The large uncertainties of the

59

extrapolations make speculation concerning observed bands of Table 7 premature.

possible correspondences

with the

The CH stretch of C&H As reported above, our search for Y, of C&H was inconclusive. Two possible candidates remain, the 3547 cm-’ and 3299 cm-’ bands. If the 3464 cm-l band is the 13C analogue of the 3547 cm-’ band then it could be ruled out as a possibility due to the large (x for the 3464 cm_, band and the large 13C shift. It is not certain, however, that these two bands are analogous. It is certain that no 13C analogue to the 3299 cm-l band has been found. From a comparison of the S/N of other 12C and 13C signals, it appears that the S/N drops by about a factor of three when using the r3C2H2 and the associated static fill method. This would result in an S/N of only about three or four for the strongest lines in a 13C analogue of the 3299 cm-l band. It seems unlikely that the CH stretch is outside the region 3250-3620 cm-’ searched, however it may be too weak for us to observe and assign. This would be most unfortunate as knowledge of this frequency is crucial to further progress in understanding the vibronic coupling. Such weakness of the CH stretch also would be very difficult to understand in view of the ab initio prediction of Reimers et al. [ 211 that the CH stretch is much stronger than the CC stretch. Assignment for other bands As mentioned above, it seems likely that the 3303 cm-’ band is the 13C analogue of the 3366 cm-’ band and the 3290 cm-l is the 13C analogue of the 3320 cm-l band. The large size of cy for the 3366 cm-’ band suggests that it is very likely to involve one quantum of the CC stretch, v3. This assumption results in an assignment of either (0,4,1) or (0,6,1) for the upper state of this band, with the latter being in better agreement with the ab initio calculations. From this, (0,6,2) c (O,O,l) seems to be a reasonable assignment for the 3320 cm - ’ and 3290 cm-’ bands. A possible assignment of the 3464 cm-’ 13C2H band is (0,0,2) t (O,O,O) because of its large (x. If the ab initio prediction for the energy of 13C2H (0,0,2) (3571 cm-‘) is multiplied by a correction factor obtained from the ratio of the observed and calculated 12C2H (O,O,l) values (1841 cm-‘/1883 cm-‘), 3491 cm-’ is obtained, in reasonable agreement with 3464 cm-‘. Although cx for the 3547 cm-’ band appears to be too small for it to be the ‘2C analogue of this band, such a possibility has not been ruled out since the reason for the severe deviation of the higher N lines from the semi-rigid rotor model for the 3547 cm-’ band is not yet understood and could possibly be affecting the observed B’.

60 ACKNOWLEDGEMENTS

We are extremely grateful to Philip Bunker for permission to publish Fig. 1 (b) and for providing preliminary vibrational energy levels from the ab initio surface.

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