High-Resolution Spectrum of the 3n 1 Band of Cyanoacetylene Obtained via Infrared/Infrared Double Resonance

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1 JOURNAL OF MOLECULAR SPECTROSCOPY 175, (1996) ARTICLE NO High-Resolution Spectrum of the 3n 1 Bnd of Cynocetylene Otined vi Infrred/Infrred Doule Resonnce Jon E. Gmogi, 1 Erik R. Th. Kerstel, 2 Xeuming Yng, 3 Kevin K. Lehmnn, nd Gicinto Scoles Chemistry Deprtment, Princeton University, Princeton, New Jersey Received August 31, 1995 The 3n 1 nd of cynocetylene hs een recorded with cm 01 resolution using sequentil infrred/infrred doule resonnce excittion. Rottionl levels from 0 J 6 were mesured nd used to clculte 0, B, nd D J. The spectrum is slightly pertured, with its fetures showing frctiontion into two or three lines due to the onset of intrmoleculr virtionl energy redistriution Acdemic Press, Inc. I. INTRODUCTION lution spectrum of the second overtone of the cetylenic CH stretch in cynocetylene (3n 1 ), using sequentil infrred/infrred doule resonnce spectroscopy. At the The spectroscopy of cynocetylene (HCCCN) hs generted quite it of interest since the detection of energy of this nd, 9700 cm 01, the density of sttes to this molecule in interstellr moleculr clouds y Turner which the 3n 1 stte cn couple is out 31 per cm 01. in 1970 (1). Techniques used include microwve (2 This stte density plces the spectrum in the intermedite 4), em mser (5), infrred (6 8), nd moleculr em regime of IVR where perturtions to the spectrum re electric resonnce (9). Hll hs studied the virtionl expected nd the perturers cn e chrcterized nd overtone nds of the n 1 mnifold of cynocetylene up sometimes identified. to 6n 1, using photocoustic lser spectroscopy (10). In the 5n 1 nd he noted loss of rottionl structure due II. EXPERIMENTAL PROCEDURES to extreme mixing. Cynocetylene is n idel molecule to chrcterize theo- Cynocetylene ws prepred y stndrd procedures reticlly ecuse it contins only five toms nd is liner. (14) nd ws premixed under helium t out 1% concen- Botschwin et l. hve clculted n initio nhrmonic trtion. A description of the high-resolution, olometerpotentil energy surfce which ccurtely predicts the known sed, moleculr em spectrometer used is given in Ref. spectrl positions of mny low-energy trnsitions (11). Using (15). Additionl detils out the doule resonnce methn empiricl fitting procedure, Hutchinson found potentil odology cn e found in Refs. (13, 16). The solute fre- energy function for stretching virtions, which firly ccuwvelength meter nd it hs n ccurcy of out quency of the spectrum is determined using home-uilt rtely reproduces high-energy overtone spectr (12). This fitted potentil ws used to clculte, y second-order perturis out cm 01. cm 01. The reltive precision of the mesured trnsitions tion theory, the eigenenergies of the 5n 1 spectrum recorded y Hll (10) nd to nlyze the sources of mixing in the spectrum (12). III. RESULTS As prt of our reserch progrm on the investigtion The 3n 1 spectrum of cynocetylene ws recorded sequenof intrmoleculr virtionl energy redistriution (IVR) tilly y first exciting fundmentl trnsition with 3.0- in cetylenic CH molecules, we hve recently studied mm color center lser, nd then exciting the corresponding severl overtone nds in dicetylene nd dicetylene- DJ Å{1, Å 3 R 1 trnsitions with 1.5-mm color center d 1 (13). Here we extend the rnge of liner molecules lser. The J Å 0 through 6 levels were mesured nd the which hve een investigted, y reporting the high-resooserved trnsitions re listed in Tle 1. Due to IVR, ech rottionl trnsition is frctionted into two or three lines 1 Current ddress: Colgte Plmolive Co., 909 River Rod, Pisctwy, (Fig. 1). Using the center of grvity of ech rottionl NJ clump, spectroscopic constnts for the 3n 1 nd hve Current ddress: Deprtment of Physics (CIO), University of Groningen, 9747 AG Groningen, The Netherlnds. een clculted (Tle 2). Using the vlue determined from microwve spectroscopy for B 0 of cm 01 3 Current ddress: Deprtment of Chemistry, University of Cliforni t (2), the Berkeley, Berkeley, CA DB for this nd cn e clculted to e 06.6(1.4) /96 $18.00 All rights of reproduction in ny form reserved. 198

2 THE 3n 1 BAND OF CYANOACETYLENE 199 TABLE 1 cm 01. This is in resonle progression with the DB s for Summry of Oserved Trnsitions ( Å 1 r 3 Step) n 1 nd 2n 1 of nd cm 01, respectively (6). The vlue of D J determined from the fit is two orders of mgnitude lrger thn the ground stte centrifugl distortion constnt. This cn e tken s n indiction of one or more ner-resonnt spectrl perturtions which tune s function of J. Qulittively the spectrum indictes the presence of two different types of perturtions. At J Å 0 the spectrum is dominted y single feture. At J Å 1 the strong line is split into two lmost equl components while t J Å 2 the second component is much weker nd y J Å 3 there is gin only one strong line. At J Å 4 the stronger feture splits gin. This rpid ppernce nd disppernce of perturing sttes indictes tht they re close in energy ut wekly coupled to the 3n 1 right stte. The two perturtions, one oserved t J Å 1, the other t J Å 4, re most likely unrelted. Since ech of these perturtions is oserved over only two J sttes, there is not enough informtion for n ccurte identifiction. A modified (17) Lwrnce Knight deconvolution (18) ws used to depertur the spectrum nd yield the energies of the perturing sttes nd their coupling strengths to the right stte. Using the depertured energies, fit ws ttempted in order to estimte the rottionl constnt of these two wek perturers. The results re listed in Tle 3. For the perturtion t J Å 1 nd 2, the 1 r 3 trnsitions of P(2), P(3), nd R(1) were used in the fit; t J Å 4 nd 5, R(3), P(6), nd R(4) were used. The DB s determined from the fit (01.77(78) nd 01.23(40) cm 01, i.e., 053 nd 037 MHz) re significntly lrger (more negtive) thn tht of the right stte. This is n indiction tht the perturing sttes contin significnt mount of stretching chrcter. At ech of the J mesured there is lso present less intense line, out 0.06 cm 01 higher in energy. Since this perturtion is present in ll the rottionl clumps, it indictes stte tht is reltively fr wy from nd more strongly coupled to the right stte. The coupling strength for this less intense line verged over ll the rottionl levels oserved is cm 01. The coupling strengths for the two perturing sttes mentioned ove re n order of mgnitude smller nd hve the vlues , , , nd cm 01 for J Å 1, 2, 4, nd 5, respectively. The results of fit to determine the spectroscopic constnts of this strongly coupled stte (fter deperturtion) re listed in Tle 4. The DB for this stte, 01.3 (4.0) cm 01 (04 MHz), is smller thn tht for the right stte, indicting some ending chrcter (/DB) in the perturer. * The wvenumers for the 0 r 1 trnsitions re from Ref. (6): R(0) The density of S / sttes round 9667 cm 01 is 31 per cm cm 01, R(1) cm 01, R(2) cm 01, R(3) This ws clculted y direct count, constrining the totl cm 01, R(5) cm 01, nd R(6) cm 01. virtionl ngulr momentum to zero. The norml modes

3 200 GAMBOGI ET AL. FIG. 1. Portions of the 3n 1 nd of cynocetylene recorded using sequentil infrred/infrred doule resonnce. The frequency scle represents the frequency of the scnning lser. nd nhrmonicities listed in Ref. (6) were used. Figure 2 shows histogrm of the DB s for ll the sttes clculted to e etween 9617 nd 9717 cm 01. The virtion rottion interction constnts, i s, used in the clcultion were o- tined from Ref. (10). The men DB is /152 MHz, indicting tht the strongly coupled perturer, with DB of 04 MHz, is considerly different thn the verge nd does not contin sttisticl mixture of ll possile perturing TABLE 2 Spectroscopic Constnts of the 3n 1 Bnd Determined from Fit of the Center of Grvities (cm 01 ) Lower stte constnts were constrined to the vlues determined in Ref. (6) for the n 1 nd: 0 Å cm 01, B 1 Å cm 01, nd D 1 Å cm 01. The vlue for B 0 ws determined from microwve spectroscopy to e cm 01 (Ref. 2). Error reported in prentheses represents 2s of the fit.

4 THE 3n 1 BAND OF CYANOACETYLENE 201 TABLE 3 Spectroscopic Constnts for the Two Depertured Wekly Coupled (Low-Frequency) Eigensttes of the 3n 1 Bnd (cm 01 ) Lower stte constnts nd D J were constrined to the vlues determined in Ref. (6) for the n 1 nd: 0 Å cm 01, B 1 Å cm 01, nd D 1 Å cm 01. The vlue for B 0 ws determined from microwve spectroscopy to e cm 01 (Ref. 2). Error reported in prentheses represents 2s of the fit. TABLE 4 Spectroscopic Constnts for the Depertured Strongly Coupled (High-Frequency) Eigenstte of the 3n 1 Bnd (cm 01 ) Lower stte constnts were constrined to the vlues determined in Ref. (6) for the n 1 nd: 0 Å cm 01, B 1 Å cm 01, nd D 1 Å cm 01. The vlue for B 0 ws determined from microwve spectroscopy to e cm 01 (Ref. 2). Error reported in prentheses represents 2s of the fit.

5 202 GAMBOGI ET AL. IV. CONCLUSIONS The 3n 1 nd of cynocetylene, with clculted density of sttes of 31 per cm 01, shows spectrum chrcteristic of the onset of IVR. There is evidence in the spectrum of two wek perturtions tht only pper riefly nd strong perturtion tht is present t ech rottionl trnsition. This high-resolution spectrum, together with other spectroscopic studies of cynocetylene, mkes this molecule good testing ground for theoreticl studies such s tht done y Hutchinson (12). REFERENCES 1. B. E. Turner, Astrophys. J. Lett. 163, L35 L39 (1971). 2. R. A. Creswell, G. Winnewisser, nd M. C. L. Gerry, J. Mol. Spectrosc. 65, (1977). 3. P. D. Mllinson nd R. L. de Zfr, Mol. Phys. 36, (1978). 4. K. T. Ymd nd R. A. Creswell, J. Mol. Spectrosc. 116, (1986). FIG. 2. Histogrm of rottionl constnts of the virtionl sttes e- 5. R. L. de Zfr, Astrophys. J. 170, (1971). tween 9617 nd 9717 cm P. D. Mllinson nd A. Fyt, Mol. Phys. 32, (1976). 7. K. Ymd, R. Schieder, G. Winnewisser, nd A. W. Mntz, Z. Nturforsch. A 35, (1980). 8. K. Ymd, R. Best, nd G. Winnewisser, Z. Nturforsch. A 38, (1984). modes. In spite of knowing the s nd mny nhrmonici- 9. R. L. DeLeon nd J. S. Muenter, J. Chem. Phys. 82, (1985). ties for the norml modes of cynocetylene, there re still 10. R. R. Hll, Ph.D. Disserttion, Rice University, P. Botschwin, M. Horn, S. Seeger, nd J. Flugge, Mol. Phys. 78, 191 too mny sttes nery in energy to ttempt definitive 198 (1993). nlysis of the perturer. If we limit the energy region to 12. J. S. Hutchinson, J. Chem. Phys. 82, (1985). {10 cm 01 nd the DB to {10 MHz, there re 28 S sttes.the 13. J. E. Gmogi, R. Z. Person, X. Yng, K. K. Lehmnn, nd G. Scoles, verge numer of qunt in this set of sttes is 12.7, com- Chem. Phys. 190, (1995). 14. F. A. Miller nd D. H. Lemmon, Spectrochim. Act Prt A 23, 1415 pred to 19.4 qunt for ll sttes within 100 cm 01. One 1423 (1967). stte out of the 28 possiilities tht is prticulrly close 15. E. R. Th. Kerstel, K. K. Lehmnn, T. F. Mentel, B. H. Pte, nd G. mtch with the oserved perturing stte hs clculted Scoles, J. Phys. Chem. 95, (1991). energy of cm 01 nd DB equl to 03.3 MHz. This 16. J. E. Gmogi, E. R. Th. Kerstel, K. K. Lehmnn, nd G. Scoles, J. Chem. Phys. 100, (1994). stte is mde up of n 2 / n 3 / n 4 / 4 n 5 / 4 n 6 virtions, 17. K. K. Lehmnn, J. Phys. Chem. 95, (1991). where n 2, n 3 nd n 4 re stretching virtions nd n 5 nd n W. D. Lwrnce nd A. E. W. Knight, J. Phys. Chem. 89, re ending virtions. (1985).