A study of the intermolecular ν51 vibration in OCH35Cl based on near infrared spectroscopy

4 July 1997

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 272 (1997) 484-488

A study o f the intermolecular v 51 vibration in O C - H on near infrared s p e c t r o s c o p y R. Garnica

a,

A.L. Mclntosh

a,

Z. Wang a, R.R. Lucchese A.R. McKellar b

a,

35C1based

John W. Bevan

a,*

a Department of Chemistry, Texas A & M Unicersity, College Station, TX 77843-3255, USA b Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ont. K1A OR6, Canada

Received 14 March 1997; in final form 24 March 1997

Abstract

The near infrared v 2 + v~ combination bands of OC-H35C1 and OC-H37CI have been recorded using a high frequency wavelength modulation diode laser supersonic jet spectrometer. In addition, the static gas phase spectra of the associated v2 + vsl_ vsi and v 2 + 2v 2 - 2v52 hot bands in OC-H35C! have been recorded using Fourier transform infrared absorption spectroscopy. The combined results permitted the evaluation of the band origin of 48.9944(2) c m - i, a rotational constant of 0.0565731(6) c m - I , a distortion constant of 1.906 × 10-7(6) cm-1, and the Coriolis coupling constant of 0.0001466(2) c m - J for the low frequency intermolecular bending vibration of the OC-H35C1 isotopomer. © 1997 Published by Elsevier Science B.V.

1. Introduction

The spectroscopy of the hydrogen-bonded complex O C - H C ! has been studied using a number o f experimental approaches. The initial work of Legon et al. [1] and Soper et al. [2] using pulsed nozzle, Fourier-transform microwave spectroscopy provided precise ground state molecular constants, a ground state molecular structure, and estimates of the high frequency v41 intermolecular bending vibration. Additional microwave and radio-frequency spectra were obtained by Altman et al. [3] using a Rabi-type molecular beam electric resonance spectrometer. The values of the v~, v 2 and v4t vibrational frequencies were later determined by Andrews et al. [4] to be

* Corresponding author.

2815.2, 2154.3 and 247.1 cm -1 respectively, from their infra-red matrix isolation study. The last frequency compares with the value of 288 c m - l estimated from microwave spectroscopy [2]. Subsequently, Bevan et al. [5,6], measured the values of u l and v 2 to be 2851.76075(13) c m - 1 and 2155.499915 (27) cm ~ respectively for the OC-H35C1 isotopomer using diode-laser and Fourier transform supersonic-jet spectroscopy and McKellar and Lu measured their respective static gas phase spectra [7] using FTIR spectroscopy. These values compared with the corresponding values of 2854, 2176 and 66 cm -~ for the v~, v 2, and u 3 vibrational frequencies predicted by Botschwina [8] using ab initio molecular orbital theory. However, there is no experimental frequency information available for the low frequency intermolecular bending fundamental vibration, v 5 in the complex although the harmonic fre-

0009-2614/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PI1 S 0 0 0 9 - 2 6 14(97)005 1 8-6

485

R. Garnica et al. / Chemical Physics Letters 272 (1997) 484-488

quency has been estimated [8] to be 56.7 cm -1. The characterization of the rovibrational energy manifold of the v~ vibration is essential data prior to initiating the modelling of the intermolecular potential of the dimer [9-12]. Direct observation of the associated transitions would be preferable using far infrared spectroscopic techniques [13,14]. However, near infrared spectroscopy can be useful in providing such information if sufficient data are produced that permit application of combination frequency differences for such purposes [15]. Specifically, observation of a combination band involving intermolecular and intramolecular vibrations in supersonic jets or molecular beams and observation of the corresponding hot band using static gas phase spectroscopy can provide such information. As the frequency of the intermolecular vibration increases, direct observation of the hot bands becomes more difficult in supersonic expansions due to state depopulation and the latter approach can be useful. We now report the rovibrational analysis of the /"2 -{- b'l combination bands of OC-H35C1 and O C H37C1 based on tunable infrared diode laser supersonic jet spectroscopy. Precise upper state combination differences can be generated from this study that provide unequivocal rovibrational assignment of the v 2 + v ~ - v~ band in OC-H35C1 observed using static gas phase FTIR spectroscopy. Extension of the latter work also permits an assignment of the v 2 + 2 v~ - 2 v~ band. When combined with the analyses from the previous studies of the v 2 O C - H C I band, we have sufficient data to evaluate rovibrational parameters for the intermolecular v~ vibration in O C - 1H 35C1.

2. Experimental The supersonic jet spectra of v 2 + v5l in both OC-H35C1 and OC-H37C1 were recorded at Texas A & M University using an upgraded tunable infrared diode laser cw supersonic jet spectrometer operating with a second derivative detection [16] frequency up to 1 MHz. The supersonic jet expansion was formed through a 12 cm long slit with 50 Ixm separation from a gaseous reservoir consisting of gases OC:HCI:Ar mixed in ratios of 8:1:91 and sustained at 15 psig. The spectrum of the complex was calibrated to an estimated absolute accuracy of 0.001

c m - ~ or better using standard frequencies of a simultaneously recorded spectrum of CO [ 17]. The static gas phase spectra were recorded using a Bomem FTIR spectrophotometer which has been described in detail elsewhere [7]. Spectra were recorded with a spectral resolution of 0.005-0.0065 cm-1 using a low temperature White cell set at 137 K with an effective pathlength of 80 m. Gas pressures were about 0.6 Torr of CO and 5 Torr of HC1. Absolute frequency calibrations of the rovibrationally resolved transitions in the isotopic species of the v 2 (CO) stretching vibration of the dimer were determined using the known frequencies of the CO monomer [17].

3. Results A segment of the rovibrationally resolved spectrum of the of v 2 + v5t in both OC-H3sC1 and OC-H37C1 is shown in Fig. 1. This spectrum has a Q branch characteristic of a 17 *- ~ band. The spectrum of the complex was recorded from approximately 2203 cm -1 to 2204.2 cm -1, and transition frequencies associated with P(4) to P(19), R(3) to R(21), and Q(13) to Q(31) of the OC-H35C1 isotopomer were fitted to Eq. (1). Corresponding transitions of P(4) to P(19), R(3) to R(21) and Q(13) to Q(31) were also used to obtain the rovibrational parameters for the OC-H37C1 isotopomer. A combination difference fit of the lower state correlates with ground state rotational constants evaluated from previous microwave and infrared investigations [ 1 - 3 , 5 7], thus confirming that the lower state of these bands is the ground state. Most importantly, this band assignment provides accurate excited state constants for the v 2 + v~ state and thus the corresponding determination of upper state combination difference frequencies for assignment purposes in other bands. Searches for the corresponding hot band v 2 + v~ - v~ in the supersonic jet expansion proved unsuccessful due to depopulation of the v~ state. The transition frequencies of the v 2 + v~ band are fitted to the expression: q' v= v o + B ' [ J ' ( J ' + 1 ) - - 12] +__-~-[J'(J'+ 1)]

- D ' j [ J ' ( J ' + 1)

-

+D')[J"(J"+ 1)] 2

12]

2 -

g"[g"(g"+ 1)] (1)

486

R. Garnica et al. / Chemical Physics Letters 272 (1997) 484-488 P(lO)

I P(IO)

P(9)

P(8)

P(7)

I

I

I

P(9)

I

I

P(8)

P(7)

I

I

P(6) P(5) I OC--H~sCI I P(6) P(5) I OC--H37C1 I

~3-

I i

i

2202.35

i

2202.45

i

i

i

2202.55

i

i

2202.65

i

2202.75

i

i

2202.85

i

i

2202.95

WAVENUMBERS(cm1) Fig. 1. A segment of the spectrum showing the P branch region of the 1)2 + 1)~ combination band in OC-HCI recorded in a supersonic jet expansion using tunable diode laser spectroscopy.

in w h i c h the ground state is fixed to the p r e v i o u s l y and precisely d e t e r m i n e d values o f B 0 and D °. The resulting fits for both 35C1 and 37C1 isotopomers are g i v e n in T a b l e 1. T h e e s t i m a t e d uncertainties reflect one standard deviation f r o m the fit. A b s o l u t e freq u e n c y accuracy is to a p p r o x i m a t e l y 0.001 c m -~. The static gas phase F T I R s p e c t r u m o f v 2 s p e c t r u m o f 16012C-1H35C1 has been assigned p r e v i o u s l y

based on m i c r o w a v e and supersoriic j e t diode laser spectroscopy [ 1 - 3 , 5 - 7 ] . As can be seen in Fig. 2, which illustrates the P branch region o f the fundamental v z and associated hot bands, this spectrum is congested, thus m a k i n g u n e q u i v o c a l a s s i g n m e n t o f the hot bands such as 1.'2 "[- /.pl - - /jl and v 2 + 2 v 2 2 ~,~ p r o b l e m a t i c without additional information. The uz + vl - v~ band is e x p e c t e d to be characterized

Table 1 I I and 1), + 21)~'-21)52 of O C - H 3 5 C 1 and where appropriate O C - H 3 7 C 1 Fitted molecular constants for the ~'2, 1)2 + vl, 1)z + 1)5-1)5, OC-H 35CI OC-H 37C1 V2 "

V2 + V~

2153.5452(2) 0.057096(7)

2155.505054(40) 0.05420894(30)

2203.54345(8) 0.05501399(45)

2 . 1 3 ( 7 ) X 10 - 7

1 . 5 5 0 ( 4 ) X 10 - 7

1.830(5) X 10 - 7

0.057359(8) 1.91(8)× 10 - 7

0.0001417(3) 0.05448403(2) 0.05448403(2) 1.5294(30) × 10- 7 1.5294(30) X 10 - 7

1)2 a

1) 2 -b 1)~

1 1 1)2 -1- 1)5--1)5

1)2

B" (cm-J ) D' (cm - l )

2155.499915(27) 0.05548062(22) 1.608(3)X 10 -7

2203.54542(6) 0.0563060(5)

2154.5510(1) 0.0563060(5)

1 . 9 2 1 8 ( 5 ) × 10 - 7

1.9218(5) X 10 - 7

q' (cm- ~) B" (cm-n) D" (cm-I) q" (cm-I ) tr (cm- i )

0.0001488(2) 0.0001488(2) 0.05576299(2) 0.05576299(2) 0.0565731(6) 1.6004(25) X 10- 7 1.6004(25)X 10 7 1.906(6) X 10 - 7 0.0001466(2) 0.0003 0.0005

~'0 (cm-j)

a Constants fixed to values from Refs. [5,6].

q- 2 V5-2 V~

0.0006

0.0003

R. Garnica et al. / Chemical Physics Letters 272 (1997) 484-488 P(34)

v2

I 101.0-

I

I

P(34)1

I P(26) I

I

P(26)

P(18)

I

I

I

I

OC--H"C~ I

I v~+v,'-v,'(~ oc__/,,cl I

I

P(28)

QC--H'CI

I v,

I

I,

I

487

,

I

v2+2V5-2vs

P(20)

I

v~+v,-v,(+e) OC--lt"Cl I= ~ I I

P(12)

I

I

I

I

I

I

I

P(20)

Q~__H35CI

I

P(28)

I

I

m Z 100.0

9,(9) 1-0 '2C'~O 98.00 i

2151.3

215~1.5

'

2151.7

215~1.9

,

, 2152.1

,

~3 21

W A V E N U M B E R S ( c m t) Fig. 2. P branch transistions of the v 2 + v s1- v 51 hot band in the spectrum of O C - H C I recorded under static gas conditions at 137 K using FTIR spectroscopy.

by a II ~ II band having paired P ( J ) and R ( J ) transitions for which the splitting is characterized by the influence of l-type doubling in both lower and upper states. To facilitate the assignment of u 2 + v1 - v~, we first searched for P ( J ) and R ( J ) branch transitions which could possibly represent such pairings and then used upper state combination difference frequencies determined from the previously discussed /"2 + /.pl analysis to confirm the assignment. Results are only presented for the OC-H35C1 isotopomer, as these were the most complete from the perspective of assignment and minimization of overlap with other transitions. Transitions P(2) to P(28) and R(3) to R(33) were used in our fits. The upper state molecular constants for v z + v 1 were fixed to those previously determined on the basis of diode jet laser spectroscopy, i.e. B~ + ~ -- 0.0565731(6), D~2+~ ] = 1.906 X 1 0 - 7 ( 6 ~ and q~2+~ = 0.0001466(2) c m - l respectively, and corresponding parameters for B~], Dx~~ and q~ and the band origin frequency v 0 fitted to the values given in Table 1. It is interesting to note that the value of B~2 - B 0 and the corresponding values of B~2+ ~ - B~] are com-

parable as expected i.e. -8.465(7) and -8.01(2) MHz, respectively. Band origins and excited state rotational and distortion constants were evaluated using the expression: q'

v= v 0 + B'[ J'( J' + 1) -/'2 ] __.-~-[ J'( J' + 1)]

- D'j[ J'( J' + l) -l'2] 2 -B"[J"(J"+l)-f --

"2]

qn

+713'(J'+

1)] + D $ [ J " ( J "

+

1)-i"2] 2

(2) with the results presented in Table 1. The combination difference associated with the band origin frequencies v 2 + v~ - v~ - (v 2 + v~) provides the corresponding band origin frequency for the low frequency intermolecular bending vibration v~. The corresponding vibrational frequency of v 1 = 48.9944(2) cm - l , thus explaining why we were unsuccessful in observing the corresponding hot bands v 2 + vI - v~ in the supersonic expansion due

488

R. Garnica et al. / Chemical Physics Letters 272 (1997) 484-488

to depopulation of the v~ state. The rotational distribution of the transitions of the v e + v~ band is consistent with a temperature of approximately 15 K. Once the band origin frequencies of the fundamental v 2 and the hot band v 2 + v~ - v~ are known, it becomes possible to estimate by extrapolation the corresponding origin of the next hot band in the series v 2 + 2 v~ - 2 v~ assuming the absence of perturbations. In addition, a corresponding monotonic behavior would be expected in the corresponding rotational constants B~2+2~ and B2~~. It was readily possible to identify a band in the gas phase spectrum with these characteristics, and we thus extend our previous assignments to the identification of the hot band v 2 + 2 v52 - 2 v~. The corresponding constants for this band are also given in Table 1, and the corresponding band origin displacement of -1.0058(2) cm -1 is almost within experimental error with X25 ( - 0 . 9 4 8 9 ( 1 ) c m - l ) . The corresponding values of B ~ , - B 0 and B2~~ - B ~ are 24.3(2) and 23.6(2) MHz respectively, which are supportive criteria of the previous assignment.

4. Conclusions The near infrared v 2 + v~ combination bands of O C - H 3 5 C 1 and O C - H 3 7 C 1 have been recorded using

a high frequency wavelength modulation diode laser supersonic jet spectrometer. In addition, the static gas phase spectra of the associated v 2 + v~ - v5l and v 2 + 2v~ - 2v~ hot bands in OC-H3sC1 have been recorded using Fourier transform infrared absorption spectroscopy. The combined results permit evaluation of rovibrational constants for the low frequency intermolecular bending vibration of the OC-H35C1 isotopomer (in cm-1): v~ = 48.9944(2); Bv~ = 0.0565731(6), D~ = 1.906 × 10-7(6) and q~ = 0.0001466(2). In the investigations currently presented, we have not resolved quadrupole substructure. Thus, we have not been able to use such information for characterization of the dynamics associated with the low frequency intermolecular vibration of the complex. The evaluated data associated with this vibration, however, gives accurate prediction of the rovibrational

parameters of the v~ vibration that will greatly facilitate such analyses using much higher resolution tunable far infrared spectrometers [13,14]. It is also possible within the perturbation approximation to estimate the anharmonic coupling constant for X25 which is approximately - 0.9489(1) c m - 1, indicating that the coupling term is quite small.

Acknowledgements We are grateful to the National Science Foundation. ALM wishes to thank the Robert A. Welch Foundation for financial support in form of the award of pre-doctoral fellowship. J. Han is thanked for his assistance in recording the diode laser spectrum. A.M. Gallegos is gratefully acknowledged for his helpful suggestions.

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