Rotationally resolved A2Π←X2Π electronic spectra of cyanodiacetyleneand dicyanoacetylene cationsWayne E. Sinclair, David Pfluger, and John P. Maier Citation: J. Chem. Phys. 111, 9600 (1999); doi: 10.1063/1.480293 View online: http://dx.doi.org/10.1063/1.480293 View Table of Contents: http://jcp.aip.org/resource/1/JCPSA6/v111/i21 Published by the AIP Publishing LLC. Additional information on J. Chem. Phys.Journal Homepage: http://jcp.aip.org/ Journal Information: http://jcp.aip.org/about/about_the_journal Top downloads: http://jcp.aip.org/features/most_downloaded Information for Authors: http://jcp.aip.org/authors

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JOURNAL OF CHEMICAL PHYSICS VOLUME 111, NUMBER 21 1 DECEMBER 1999

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Rotationally resolved A 2P—X 2P electronic spectra of cyanodiacetyleneand dicyanoacetylene cations

Wayne E. Sinclair, David Pfluger, and John P. MaierInstitute for Physical Chemistry, University of Basel, Klingelbergstrasse 80, CH 4056 Basel, Switzerland

~Received 18 May 1999; accepted 10 August 1999!

The 000 bands of theA 2P←X 2P and A 2Pg←X 2Pu transitions of cyanodiacetylene and

dicyanoacetylene cations have been rotationally analyzed at Doppler-limited resolution usingfrequency modulation absorption spectroscopy. The ions were generated in a liquid-nitrogen cooledhollow cathode modulated discharge incorporated in a White cell. Analysis of the rotationalstructure yields molecular parameters of the cations in the two electronic states, confirming lineargeometries. The inferred spin–orbit and rotational constants are discussed in terms of the electronicstructure of the isoelectronic cations. The spectrum of dicyanoacetylene cation revealsL-typedoubling consistent with the presence of a nearby2Sg

1 state. The second order spin-orbit splittingof the A 2Pg state is analyzed to predict the existence of the2Sg

1 state below theA 2Pg state.© 1999 American Institute of Physics.@S0021-9606~99!01441-5#

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I. INTRODUCTION

The cyanopolyacetylenes are known to be presendark interstellar clouds.1 The identification of HC11N, thelargest known molecule to be detected in the interstellardium by radio astronomy, has been established, followinglaboratory identification.2 The dicyano derivatives are unsuiable for microwave detection but may well be comparaabundant. Following the detection of the polar cyanopoacetylenes, ion–molecule reactions have been considerethe processes leading to their formation in such envirments.3

In view of the above, the role of such species in diffuinterstellar clouds and possible relation to the diffuse intstellar band~DIB! absorptions is to be considered. In fact tsuggestion of Douglas that small carbon chains are gcandidates for DIBs~Ref. 4! was stimulated by the detectioof cyanopolyacetylenes in dark clouds. There is a repodetection of HC5N and HC7N in diffuse media in absorptionby microwave spectroscopy.5 Thus the electronic spectra osuch species are of interest. However, whereas the nespecies will have their electronic transitions in the UV, tcations absorb in the range of the DIB observations~;400–900 nm!. Thus the measurement of the electronic spectrathe gas phase in the laboratory is a goal. Especially ifrotational structure can be resolved and the spectroscconstants determined then the band profiles can be predat the appropriate temperature and compared to the anomical data. Moreover, their linear structure and extenp conjugated systems make them ideal for the investigaof molecular electronic structure and bonding theory.

To date, gas phase studies of cyanopolyacetylene caare limited. Emission and laser-induced fluorescence spehave been obtained for NC4N

1 ~Refs. 6, 7! and gas phaseemission spectra have been observed for HC5N

1 andNC6N

1.8,9 Fluorescence could not be detected for NC2N1

and HC3N1 due to efficient intramolecular processes.10 In

9600021-9606/99/111(21)/9600/9/$15.00

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addition, a number of studies of the electronic transitionsneon matrices using laser excitation of fluorescen(NC2nN1, n52 – 4 and HC2n11N1, n52,3! ~Refs. 11–13!or direct absorption~NC2nN1, n52 – 6 and HC2n11N1, n52 – 6! ~Ref. 14! have been reported. In this paper, a dtailed study of the rotationally resolvedA 2P←X 2P 00

0

electronic transition of the cyanodiacetylene cation HC5N1,

its isotopic species DC5N1, and of theA 2Pg←X 2Pu 00

0

electronic transition of the dicyanoacetylene cation NC4N1

is presented.The A 2P→X 2P gas phase emission spectrum

HC5N1 was observed employing low energy electron be

excitation in an effusive jet.8 The A 2P state lifetime andfour totally symmetric vibrational fundamentals in theX 2Pstate were determined from this study. The 00

0 transition at17 19065 cm21 appeared as one band at an experimenoptical resolution of 0.18 nm. The emission spectrumNC4N

1 was also observed by the same method.6 In contrastto HC5N

1, the symmetry of theA state proved more difficultto assign. The emission band system was initially attributhe A 2Sg

1→X 2Pu transition based on the calculations bthe Greens function method which indicated that three st2Pg , 2Sg , and2Su are nearly degenerate.15 In a later study,the transition was examined by photoelectron–photon cocidence measurements and by its laser excitation spectr7

Based on a comparison of the determined radiative rate wthat of other organic cations, a2Pg symmetry was inferredfor the A state.

The combined techniques of frequency modulation~FM!absorption spectroscopy and discharge modulation, couwith phase sensitive detection, has been shown to be asitive method to detect transient species. The methodused to obtain the Doppler-limitedA 2Pg←X 2Pu 00

0 elec-tronic spectrum of the triacetylene cation (HC6H

1) gener-ated in a hollow cathode discharge fed on acetylene.16 In thepresent work, the technique is used to obtain the Dopp

0 © 1999 American Institute of Physics

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9601J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Electronic spectra of cyanopolyacetylenes cations

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FIG. 1. Rotationally resolved FM absorption spectra of theA 2P←X 2P 000 band of HC5N

1 and theA 2Pg←X 2Pu 000 band of NC4N

1 in a liquid-nitrogencooled hollow cathode discharge of a mixture of C2H2 and C2N2 in He. Also shown are portions of the observed spectra with corresponding rotatassignments. The instrumental lineshape is approximately first derivative, due to the modulation scheme employed.

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limited A 2P←X 2P 000 absorption spectrum of the cyanod

acetylene cation HC5N1, its isotopic species DC5N

1, andthe A 2Pg←X 2Pu 00

0 absorption spectrum of the dicyanoacetylene cation NC4N

1. The analyses of the fully resolved rotational structure yield the first accurate valuesthe rotational and spin–orbit constants for the groundexcited states of cyanopolyacetylene cations. Perturbationthe A 2Pg state of NC4N

1 have been studied in detail anare discussed in terms of its interaction with the nearby2(g

1

state.

II. EXPERIMENT

The experimental apparatus used to record direct abstion measurements of transient species from a combinaof frequency modulation~FM! spectroscopy and hollowcathode discharge modulation has been described.16 The ionsof interest were produced in a liquid nitrogen-cooled hollocathode discharge cell mounted inside a 2 m White-typemultireflection absorption cell. The 17 kHz ac voltage wamplified by a 1.2 kW amplifier, stepped-up by a tranformer, rectified and applied to the anodes while the cathwas kept at earth. A mixture of acetylene (C2H2) and cyano-gen ~C2N2, 0.5% each! in He was used for generatinHC5N

1 and NC4N1. Deuterated acetylene was the precur

for generating DC5N1. A total pressure of 1.0–1.5 mbar wa

maintained in the cell with the discharge current of 200 mpeak to peak. A single mode ring dye laser pumped by aW cw Ar1 laser was phase-modulated at a radio frequenc192 MHz by an electro-optic modulator~EOM!. Radio fre-quency power into the modulator was adjusted to give fi

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order sidebands typically 25% of the carrier frequency intsity as monitored by a Fabry–Perot e´talon spectral analyzerThe frequency-modulated laser beam was passed througdischarge cell and focused onto a fast photodiode. The hfrequency components of the photodiode were amplifieddemodulated in a double balanced mixer, referenced tolocal radio frequency source, which drives the EOM. Tphotodiode signal was fed to a lock-in amplifier, which dmodulated the signal at the discharge modulation frequenA PC with an autoscan system software served to collectdata and to record the spectra. Absolute frequency calition was performed by simultaneously recording the I2 ab-sorption spectrum with an estimated accuracy of better t0.005 cm21. FM signal is an approximate first derivative othe absorption line resulting from the fact that the sidebasplitting is comparable to the Doppler width of the rovbronic transitions. The resulting rotationally resolved specwere analyzed using the simulation and least squares fitprogram Pgopher.17

III. RESULTS

The high resolution FM absorption spectra of the00

bands of HC5N1 and NC4N

1 identified in previous emissionstudies are displayed in the upper traces of Fig. 1. The stra show very different qualitative features. However, thhave several common characteristics. Both bands are cposed of two subbands each comprising a strong, wdefinedP and R branch extending away from a relativeweak Q branch located near the origin. They are consistwith parallel (DV50) bands and are assigned to theA 2P

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9602 J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Sinclair, Pfluger, and Maier

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←X 2P (HC5N1) and A 2Pg←X 2Pu (NC4N

1) transitions.The rotational structure is red-shaded indicating thatB08,B09 . Portions of the spectra are expanded in the lowtraces and exhibit clearly resolved rotational lines with sinrotational linewidths@full-width at half-maximum~FWHM!#of roughly 500 MHz, corresponding to a translational teperature of about 150 K in the cell.

The subbands are attributed to theV51/2 andV53/2spin–orbit components separated by the difference inspin–orbit interaction constant for each electronic state,uA092A08u. The HC5N

1~NC4N1! cation is described by the con

figurations in theX 2P(X 2Pu) and A 2P(A 2Pg) states as...1p42p3(...1pg

42pu3) and ...1p32p4(...1pg

32pu4), respec-

tively. Both states are inverted, i.e., theV53/2 component islower lying than theV51/2 one, withA0 having a negativevalue. The more intense subband correspond to theV53/2component due to the higher population of this level.

The 000 band of HC5N

1 was recorded between 17 16and 17 185 cm21. The two R branches form band headsJ'78.5 with a separation between the branch maximaabout 3 cm21. No L-type splitting of the rotational lines waobserved within the experimental resolution. The higher fquency subband has more intensity and is assigned toV53/2 component. This assignment was confirmed by atermination of the rotational numbering of the lines at tband center near theQ3/2 branch head observed at 17 179cm21. The lower trace of Fig. 1 shows the part of the sptrum in this region. The subband exhibits a band gap'10B, establishing unambiguously that the first transitioin the P and R branches have the line numbersP(2.5) andR(1.5). TheQ1/2 branch of the lower-lying band is burieunder the rotational structure of the higher-lying subbaThe corresponding transition of DC5N

1 exhibited similarqualitative features with an isotopic shift of110.9 cm21 andwas recorded between 17 174 and 17 191 cm21.

The 000 band of NC4N

1 was recorded between 16 76and 16 785 cm21. In contrast to the spectrum of HC5N

1, thesubbands are closely overlapping. Intensity oscillationsobserved as the line spacing between the two subbandscome commensurate or not. The lower trace of Fig. 1 sha part of the spectrum. Moreover, theR branch band headare very weak compared to HC5N

1. The lower lying sub-band has more intensity and is assigned to theV53/2 sub-band. TheP branch of theV53/2 component atJ.43.5shows the onset of pronounced staggering. This perturbais characteristic ofL-type doubling with consideration of thnuclear spin statistics for NC4N

1. At higher values ofJ thetwo P branches were fully resolved into two distinct setstransitions, corresponding to thee and f L-type doublingcomponents of theP-branch of theV53/2 component. Thisis seen in Fig. 2 for the resolvedL doubling ofP(73.5) andP(74.5). The even:odd intensity alternation is observedthe ratio 2:1 consistent with identical N14 nuclei with spinI 51 and provides an unambiguous assignment of thee andfcomponents. Thee levels are observed to be at lower enerthan thef levels. On the other hand, a small staggering ofP branch of theV51/2 component was detected at muhigher values ofJ. However, noL-doublets were observedirectly as they were largely overlapped with theV53/2

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component and due to several unidentified lines. Frommagnitude of the observed effects it was evident thatA 2Pg state is significantly perturbed with respect to tX 2Pu state.

The resulting absolute numbering of the rotational linin the spectra are indicated partly in the lower traces of F1. The 00

0 A 2P←X 2P band of HC5N1 (DC5N

1) andNC4N

1 exhibit 183 ~176! and 182 assigned transitions, rspectively. Rotational analysis of the spectra was performby standard procedures. The line positions were fit witheffective rotational Hamiltonian where both spin–orbit sytems were considered simultaneously. In this, the band or(n0), the rotational constant (B0), centrifugal distortion con-stant (D0), and spin–orbit interaction constant (A0) of bothstates were adjustable parameters. In the case of NC4N

1 itwas necessary to include theL-type doubling constants~p0

and q0!. To reflect the perturbation of theA 2P state, thevalues ofp08 andq08 of the A 2P state were adjusted, whilethose of theX 2P state were fixed to zero. The line positionof the observed lines in the 00

0 bands of HC5N1 and DC5N

1

could be fit to standard deviations that are significantly lthan the observed linewidths. The observed line positiand residuals are gathered in Tables I–III.

The optimal values of the molecular constants are shoin Table IV. The derived rotational constants of HC5N

1 andNC4N

1 in the X 2P states are comparable to those of tcorresponding neutral ones;B0~HC5N!50.044 408 cm21 andB0~NC4N!50.044 587 cm21.18,19TheD0 parameters were onthe order of 1029, but had uncertainties as large as the valuobtained and are not shown in Table IV. Using Kraitchmaequations,20 the hydrogen atom position in the center-omass system of HC5N

1 was determined from a comparisoof the observed rotational constants of HC5N

1 and DC5N1.

The derived value in theX 2P state is 42361 pm, compa-

FIG. 2. ~A! ResolvedL-type doubling of theJ573.5 and 74.5P branchlines of NC4N

1 in the V53/2 component. The vertical lines show thpositions and relative intensity of theL-type doubling components calculated from the molecular constants in Table IV.~B! Corresponding energylevels of theX 2Pu(V53/2) andA 2Pg(V53/2) states of NC4N

1, indicat-ing the paritye/ f of each state.

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9603J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Electronic spectra of cyanopolyacetylenes cations

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TABLE I. Frequencies~in cm21! and assignments for theA 2P←X 2P 000 absorption spectrum of HC5N

1 and residuals (obs2calc) from the fit.

J Obs o2ca J Obs o2ca J Obs o2ca J Obs o2ca J Obs o2ca J Obs o2ca

R3/2(J) P3/2(J) R1/2(J) P1/2(J)

1.5 17179.821 3 2.5 17179.376 0 51.5 17173.00721 7.5 17177.176 0 3.5 17176.160 1 58.5 17168.6812.5 17179.900 22 3.5 17179.284 0 52.5 17172.840 1 10.5 17177.398 2 8.5 17175.66921 59.5 17168.502 213.5 17179.984 0 4.5 17179.189 1 55.5 17172.325 0 11.5 17177.469 3 9.5 17175.568 0 62.5 17167.9574.5 17180.064 21 5.5 17179.093 0 56.5 17172.150 0 12.5 17177.536 1 10.5 17175.46222 63.5 17167.771 215.5 17180.145 1 9.5 17178.696 3 57.5 17171.975 1 14.5 17177.66523 12.5 17175.253 1 64.5 17167.586 06.5 17180.223 2 10.5 17178.591 2 60.5 17171.437 1 15.5 17177.732 1 13.5 17175.14122 65.5 17167.397 217.5 17180.300 3 17.5 17177.81921 61.5 17171.255 1 17.5 17177.85521 14.5 17175.032 22 66.5 17167.210 18.5 17180.374 2 18.5 17177.704 1 62.5 17171.06921 19.5 17177.974 1 16.5 17174.80623 67.5 17167.017 219.5 17180.447 3 19.5 17177.586 0 65.5 17170.513 3 21.5 17178.087 2 17.5 17174.68925 69.5 17166.634 3

10.5 17180.516 0 20.5 17177.469 2 66.5 17170.31821 23.5 17178.194 4 18.5 17174.578 0 68.5 17166.8252111.5 17180.585 21 21.5 17177.346 0 67.5 17170.131 3 24.5 17178.23822 20.5 17174.340 21 71.5 17166.237 2312.5 17180.654 0 22.5 17177.22221 68.5 17169.935 0 26.5 17178.341 5 21.5 17174.21922 72.5 17166.038 2313.5 17180.721 0 23.5 17177.100 0 70.5 17169.548 4 28.5 17178.428 2 22.5 17174.09821 73.5 17165.839 2214.5 17180.785 21 24.5 17176.975 1 71.5 17169.34522 30.5 17178.511 0 24.5 17173.84821 74.5 17165.639 015.5 17180.851 2 25.5 17176.851 4 72.5 17169.150 2 31.5 17178.553 3 25.5 17173.723 1 75.5 17165.4342216.5 17180.913 1 26.5 17176.722 3 73.5 17168.949 2 32.5 17178.591 3 26.5 17173.595 1 76.5 17165.23417.5 17180.971 21 27.5 17176.589 21 74.5 17168.745 0 33.5 17178.627 2 27.5 17173.46124 77.5 17165.024 2118.5 17181.029 22 28.5 17176.457 21 75.5 17168.543 3 34.5 17178.662 2 29.5 17173.200 0 79.5 17164.60819.5 17181.087 23 29.5 17176.325 0 78.5 17167.91921 35.5 17178.696 3 30.5 17173.067 121.5 17181.197 22 30.5 17176.192 1 79.5 17167.713 3 36.5 17178.727 2 31.5 17172.933 322.5 17181.251 22 31.5 17176.057 3 80.5 17167.499 1 37.5 17178.756 1 32.5 17172.7922123.5 17181.301 22 32.5 17175.915 21 81.5 17167.284 22 40.5 17178.837 21 35.5 17172.368 2324.5 17181.350 24 33.5 17175.776 21 83.5 17166.855 0 41.5 17178.865 2 36.5 17172.2242325.5 17181.398 24 34.5 17175.636 21 86.5 17166.196 22 37.5 17172.080 2226.5 17181.446 23 35.5 17175.494 0 87.5 17165.976 1 38.5 17171.935 027.5 17181.489 24 36.5 17175.355 4 88.5 17165.75021 41.5 17171.487 128.5 17181.536 22 37.5 17175.206 2 89.5 17165.52521 42.5 17171.335 229.5 17181.578 22 38.5 17175.059 1 90.5 17165.300 1 43.5 17171.1782130.5 17181.620 21 39.5 17174.912 2 44.5 17171.0222131.5 17181.660 1 40.5 17174.761 2 Q1/2(J) 47.5 17170.548 132.5 17181.698 1 41.5 17174.611 3 48.5 17170.386 233.5 17181.730 22 42.5 17174.455 0 1.5 17179.598 3 49.5 17170.221 034.5 17181.764 23 43.5 17174.300 21 50.5 17170.053 2335.5 17181.798 22 44.5 17174.141 23 51.5 17169.889 036.5 17181.830 22 45.5 17173.984 22 54.5 17169.382 140.5 17181.940 21 46.5 17173.824 23 55.5 17169.207 2141.5 17181.965 1 47.5 17173.668 1 56.5 17169.036 1

48.5 17173.502 22 57.5 17168.860 050.5 17173.171 24

a(obs2calc) in units of 1023 cm21.

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rable with the value in the ground state (1S1) of the neutralmolecule~425 pm!.18 Excitation to theA 2P (HC5N

1) andA 2Pg (NC4N

1) states produces a change in theB0 rota-tional constant,DB5(B082B09), of 20.000 76 and20.000 5cm21, respectively. A similar analysis suggest that the hdrogen atom position in HC5N

1 increases in theA 2P stateto 42561 pm.

The derived values ofA08 and A09 are highly correlatedand have relatively larger error bars. The valueA09~HC5N

1! is derived to be roughly236 cm21. Deuteriumsubstitution has little effect on the magnitude ofA09 . Due tothe perturbations in theA 2Pg state of NC4N

1, the value ofA09 was less well determined. A more reliable estimate canobtained by assuming that every replacement of a CH bin HC6H

1 results in a linear increment ofA09 . The value ofA09 for HC6H

1 was estimated to be about231 cm21.16 Thevalue for HC5N

1 adds'25 cm21 to the value for HC6H1.

Therefore, the value ofA0(X 2Pu) for NC4N1 is estimated

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to be about241 cm21. This is a reasonable assumptiogiven that this value compares well with the valueA0(X 2P)540.34 cm21 for the CCN radical.21

The values ofDA5(A082A09) are accurately determinedThe value ofDA for HC5N

1 and NC4N1 are derived to be

23.12 and 0.53 cm21, respectively. The derived values op08(A

2Pg) and q08(A2Pg) are '431024 and 2431025,

respectively for NC4N1. The opposite signs are consiste

with an inverted2P state.

IV. DISCUSSION

In order to gain insight into the contrasting observatiofor the 2P←2P transitions of isoelectronic HC5N

1 andNC4N

1, the molecular parameters are compared with thof isoelectronic HC6H

1. The molecular constants derivefrom the analysis16 of theA 2Pg←X 2Pu 00

0 band of HC6H1

are shown in Table IV. The transitions for HC5N1 and

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9604 J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Sinclair, Pfluger, and Maier

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TABLE II. Frequencies~in cm21! and assignments for theA 2P←X 2P 000 absorption spectrum of DC5N

1 and residuals (obs2calc) from the fit.

J Obs o2ca J Obs o2ca J Obs o2ca J Obs o2ca J Obs o2ca J Obs o2ca

R3/2(J) P3/2(J) R1/2(J) P1/2(J)

1.5 17190.257 22 3.5 17189.750 2 61.5 17182.099 0 7.5 17187.626 0 8.5 17186.189 1 63.5 17178.663212.5 17190.329 4.5 17189.662 4 64.5 17181.574 5 10.5 17187.837 0 9.5 17186.090 0 64.5 17178.485223.5 17190.418 1 5.5 17189.570 4 65.5 17181.391 1 12.5 17187.96922 11.5 17185.888 22 67.5 17177.941 244.5 17190.496 1 6.5 17189.476 3 66.5 17181.210 1 14.5 17188.097 0 12.5 17185.78622 68.5 17177.760 225.5 17190.570 0 18.5 17188.242 2 67.5 17181.028 1 17.5 17188.277 0 13.5 17185.687 2 69.5 17177.574246.5 17190.645 1 19.5 17188.131 2 69.5 17180.65721 19.5 17188.387 23 15.5 17185.474 0 70.5 17177.388237.5 17190.720 2 21.5 17187.902 3 71.5 17180.287 4 21.5 17188.49522 16.5 17185.366 1 71.5 17177.204 08.5 17190.790 2 22.5 17187.783 0 72.5 17180.09122 23.5 17188.596 21 17.5 17185.253 23 72.5 17177.015 09.5 17190.858 21 23.5 17187.664 21 73.5 17179.906 3 24.5 17188.650 4 19.5 17185.03123 73.5 17176.821 23

10.5 17190.927 1 24.5 17187.548 3 76.5 17179.321 1 26.5 17188.73821 20.5 17184.920 21 76.5 17176.246 311.5 17190.992 21 25.5 17187.428 3 77.5 17179.126 2 27.5 17188.786 3 21.5 17184.80223 77.5 17176.049 212.5 17191.058 21 26.5 17187.303 1 78.5 17178.928 3 28.5 17188.827 2 23.5 17184.56922 78.5 17175.848 2113.5 17191.123 0 27.5 17187.179 1 79.5 17178.725 0 31.5 17188.947 4 24.5 17184.451 014.5 17191.186 1 28.5 17187.05321 80.5 17178.522 22 34.5 17189.049 21 25.5 17184.326 2415.5 17191.248 2 29.5 17186.92521 83.5 17177.909 22 36.5 17189.112 0 26.5 17184.2072116.5 17191.306 0 30.5 17186.800 2 84.5 17177.70321 37.5 17189.142 1 28.5 17183.960 117.5 17191.363 21 31.5 17186.669 0 85.5 17177.49322 29.5 17183.832 2118.5 17191.421 0 32.5 17186.53522 86.5 17177.285 1 30.5 17183.707 319.5 17191.476 0 33.5 17186.40222 87.5 17177.073 0 31.5 17183.5732220.5 17191.532 3 34.5 17186.27021 88.5 17176.861 1 32.5 17183.445 121.5 17191.583 1 35.5 17186.13322 91.5 17176.212 0 33.5 17183.314 222.5 17191.633 1 36.5 17185.99623 92.5 17175.993 0 34.5 17183.178 023.5 17191.682 0 37.5 17185.85822 35.5 17183.042 2124.5 17191.729 21 38.5 17185.720 0 Q3/2(J) 36.5 17182.910 425.5 17191.777 1 40.5 17185.435 0 39.5 17182.488 226.5 17191.821 21 41.5 17185.291 21 1.5 17190.052 2 40.5 17182.3432127.5 17191.863 22 42.5 17185.147 2 41.5 17182.201 128.5 17191.905 22 43.5 17184.996 23 42.5 17182.054 029.5 17191.945 22 44.5 17184.848 22 45.5 17181.612 330.5 17191.985 21 46.5 17184.545 24 46.5 17181.460 231.5 17192.023 21 47.5 17184.393 22 47.5 17181.307 232.5 17192.057 23 48.5 17184.238 23 48.5 17181.153 233.5 17192.092 23 49.5 17184.083 21 49.5 17180.996 134.5 17192.127 21 50.5 17183.926 22 52.5 17180.519 2135.5 17192.159 21 51.5 17183.768 0 53.5 17180.361 336.5 17192.189 21 52.5 17183.608 0 54.5 17180.199 337.5 17192.221 3 53.5 17183.44521 55.5 17180.029 2238.5 17192.244 21 54.5 17183.281 22 56.5 17179.868 339.5 17192.272 1 55.5 17183.118 0 59.5 17179.359 040.5 17192.297 1 56.5 17182.953 1 60.5 17179.1872141.5 17192.320 1 59.5 17182.444 0 61.5 17179.015 142.5 17192.340 0 60.5 17182.274 2 62.5 17178.840 0

a(obs2calc) in units of 1023 cm21.

ic-

r

eo

lf

t re-hed

y

NC4N1 are observed at 523 and 125 cm21, respectively, to

higher energy than the transition for HC6H1. The species

have very similar rotational constants. Notably, HC5N1 and

HC6H1 exhibit comparable values ofDB and DA and thus

the overall appearance of their 000 bands are qualitatively

similar.The molecular orbital descriptions of the electron

structure for HC5N1 and NC4N

1 are expected to be qualitatively similar to HC6H

1. However, they differ in the distri-bution of electron charge due to the fact that N is moelectronegative than C. The derived values ofB0(X 2Pu) andB0(A 2Pg) for HC6H

1 were found to be consistent with thmolecular orbital considerations of the electronic structurethe neutral molecule. Neutral HC6H has the molecular orbitaconfiguration ...1pg

42pu4. The qualitative representation o

the molecular orbital descriptions of HC6H and its ion are

ownloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstrac

e

f

shown below~the shaded and unshaded lobes represengions of the opposite sign of the wave function and daslines indicate nodes!.

The X 2Pu state of HC6H1 is adequately described b

the single configuration ...1pg42pu

3. As the (2pu) orbital isbonding in the CwC regions but antibonding in the C–C

t. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

2

6

9605J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Electronic spectra of cyanopolyacetylenes cations

D

TABLE III. Frequencies~in cm21! and assignments for theA 2Pg←X 2Pu 000 absorption spectrum of NC4N

1 and residuals (o2c) from the fit.

J Obs o2ca J Obs o2ca Jb Obs o2ca Jb Obs o2ca J Obs o2ca

R3/2(J) P3/2(J) P1/2(J)

1.5 16780.260 26 2.5 16779.821 22 44.5f 16775.074 4 76.5f 16770.255 21 4.5 16780.159 12.5 16780.355 3 3.5 16779.732 2 45.5e 16774.929 3 76.5e 16770.200 1 6.5 16779.970 13.5 16780.438 1 4.5 16779.63721 46.5f 16774.803 1 77.5f 16770.087 22 7.5 16779.873 14.5 16780.520 21 5.5 16779.543 21 47.5e 16774.655 2 77.5e 17770.030 21 8.5 16779.777 25.5 16780.601 22 6.5 16779.448 21 48.5f 16774.521 26 78.5f 16769.923 2 10.5 16779.578 06.5 16780.687 2 7.5 16779.354 1 49.5e 16774.370 23 78.5e 16769.857 23 11.5 16779.480 37.5 16780.768 3 8.5 16779.25224 50.5f 16774.248 0 79.5f 16769.752 0 12.5 16779.378 38.5 16780.849 5 9.5 16779.158 0 51.5e 16774.089 21 79.5e 16769.687 23 15.5 16779.060 259.5 16780.925 2 10.5 16779.06021 52.5f 16773.964 21 80.5e 16769.581 21 16.5 16778.959 21

10.5 16781.001 1 11.5 16778.959 0 53.5e 16773.801 22 81.5f 16769.342 22 17.5 16778.856 311.5 16781.077 0 12.5 16778.85622 54.5f 16773.680 2 18.5 16778.751 612.5 16781.151 21 13.5 16778.751 24 55.5e 16773.510 21 Q3/2(J) 21.5 16778.414 2213.5 16781.227 1 14.5 16778.64725 56.5f 16773.388 0 1.5 16780.046 0 22.5 16778.30514.5 16781.300 0 15.5 16778.54423 57.5e 16773.213 22 23.5 16778.193 215.5 16781.370 22 16.5 16778.439 23 58.5f 16773.094 3 24.5 16778.082 516.5 16781.443 0 17.5 16778.33224 59.5e 16772.913 23 25.5 16777.961 017.5 16781.519 6 18.5 16778.22623 60.5f 16772.791 22 R1/2(J) 26.5 16777.845 2118.5 16781.584 2 19.5 16778.120 0 60.5e 16772.761 23 7.5 16781.300 4 27.5 16777.730 119.5 16781.648 22 20.5 16778.010 21 61.5f 16772.631 0 16.5 16781.975 24 28.5 16777.608 2320.5 16781.715 22 21.5 16777.898 22 61.5e 16772.613 1 22.5 16782.385 0 29.5 16777.4902121.5 16781.785 1 22.5 16777.78523 62.5f 16772.491 2 23.5 16782.440 23 30.5 16777.369 2222.5 16781.849 1 23.5 16777.67224 63.5e 16772.305 2 27.5 16782.689 5 31.5 16777.25523.5 16781.911 21 24.5 16777.561 21 64.5f 16772.184 2 32.5 16777.129 124.5 16781.978 3 25.5 16777.445 0 64.5e 16772.147 21 33.5 16777.007 325.5 16782.037 0 26.5 16777.33022 65.5f 16772.028 1 37.5 16776.4902526.5 16782.096 21 27.5 16777.214 21 65.5e 16771.992 1 38.5 16776.3682427.5 16782.154 23 28.5 16777.096 21 66.5f 16771.874 3 39.5 16776.239 028.5 16782.216 0 29.5 16776.978 0 67.5f 16771.716 2 44.5 16775.5702329.5 16782.278 4 30.5 16776.864 6 67.5e 16771.674 21 45.5 16775.432 530.5 16782.333 2 31.5 16776.743 6 68.5f 16771.560 4 46.5 16775.299 031.5 16782.384 22 32.5 16776.618 3 68.5e 16771.516 1 47.5 16775.164 332.5 16782.441 0 33.5 16776.495 3 69.5f 16771.400 4 48.5 16775.0172333.5 16782.494 0 34.5 16776.36821 69.5e 16771.354 0 49.5 16774.882 035.5 16782.595 23 35.5 16776.242 21 70.5f 16771.238 1 50.5 16774.741 336.5 16782.649 0 36.5 16776.117 0 70.5e 16771.195 3 51.5 16774.602 337.5 16782.694 24 37.5 16775.986 24 71.5e 16771.031 1 52.5 16774.452 138.5 16782.747 1 38.5 16775.862 1 72.5f 16770.911 23 53.5 16774.310 2139.5 16782.794 0 39.5 16775.73022 73.5f 16770.750 21 54.5 16774.165 540.5 16782.844 4 40.5 16775.604 2 73.5e 16770.700 21 55.5 16774.014 2541.5 16782.885 0 41.5 16775.46724 74.5f 16770.585 22 56.5 16773.870 542.5 16782.933 3 42.5 16775.340 2 74.5e 16770.537 3 57.5 16773.7232143.5 16782.971 22 43.5 16775.202 22 75.5e 16770.367 21

a(o2c) in units of 1023 cm21.be and f designate transitions to lower and higherL-type doubling components, respectively.

censu

s

y

c

in

-

r-

regions, the CwC distances increase but the C–C distandecrease relative to the neutral molecule values. As a coquence of these effects, a net contraction of the molecarises yielding a larger value ofB0 than for the neutral mol-ecule; B0(1Sg

1)2B0(X 2Pu)520.0042 cm21. The reduc-tion in antibonding character of the C–C bonds appeardominate the structural changes in HC6H upon ionization tothe X 2Pu state.

The A 2Pg state of HC6H1 corresponds predominantl

to the configuration ...1pg32pu

4. The (1pg) orbital isstrongly bonding in the outer CwC regions and slightlybonding in the C–C regions. The outer CwC distances in-crease relative to the neutral values yielding a smallerB0

value than for the neutral molecule;B0(1Sg1)2B0(A 2Pg)

50.0038 cm21. In this case the reduction in bonding chara

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se-le

to

-

ter of the CwC bonds determines the structural changesHC6H upon ionization to theA 2Pg state.

Table V compares the values ofB0(1Sg1)

2B0(X2Pu ,A 2Pg) derived for HC6H

1 to the correspondingvalues for HC5N

1 and NC4N1. Notably, the table indicates

that theuB0(1Sg1)2B0(X 2Pu)u values are observed to de

crease monotonically in the sequence HC6H1, HC5N

1,NC4N

1. The uB0(1Sg1)2B0(A 2Pg)u value is largest for

HC5N1 while it is comparable for HC6H

1 and NC4N1.

Qualitatively, the values for HC5N1 and NC4N

1 are similarto the values observed for HC6H

1. The B0(X 2Pu) valuesare larger than those ofB0(1Sg

1), while theB0(A 2Pg) val-ues are smaller. This result is consistent with HC6H

1,HC5N

1, and NC4N1 ions having analogous molecular o

bital descriptions of the electronic structure.

t. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

9606 J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Sinclair, Pfluger, and Maier

Downloaded 13 Sep 2013 to

TABLE IV. Molecular constants~in cm21! of HC5N1, DC5N

1, NC4N1, and HC6H

1 in the vibrationless levelsof theX 2P andA 2P states, derived from analysis of the rotationally resolvedA 2P←X 2P absorption spectra.N is the number of lines used in the fit; o2c is the observed minus calculated standard deviation of the fit~incm21!.a

HC5N1 DC5N

1 NC4N1 HC6H

1b

X 2P state X 2Pu state X 2Pu state

B09 0.044 616 5~67! 0.042 629 0~73! 0.044 687~15! 0.044 594 3~34!A09 235.71~41! 236.47~57! 245~10! 231.40~28!

A 2P state A 2Pg state A 2Pg state

B08 0.043 848 9~68! 0.041 901 3~71! 0.044 166~18! 0.043 792 1~34!A08 232.58~41! 233.38~57! 245~10! 228.41~28!p08 3.47(29)31024

q08 23.84(15)31025

DB 20.000 76 20.000 72 20.0005 20.000 80DA 23.12 23.09 0.53 22.99n0 17 178.039 51~27! 17 188.505 41~30! 16 780.311 78~49! 16 654.687 26~25!N 183 176 182 277o2c 0.0019 0.0021 0.0031 0.0024

aOne standard deviation given in parentheses.bReference 16.

nstr

-th

egon

gt

n

eio

io

for

arheitof

l C

fng

w-

e

id-onic

l.-

ve.tstion

n

ofthe

The trends observed in Table V must in the broad sebe correlated with differences in the electron charge disbution. The structural changes for neutral HC6H that occurupon ionization to theX 2Pu state are dominated by the reduction in antibonding character of the C–C bonds. Incorresponding molecular orbital for neutral HC5N andNC4N, the CwN group withdraws electron density from thcentral CwC bonding region, reducing the anti-bondincharacter of the adjacent C–C bond. Therefore, ionizatito theX 2P states of HC6H, HC5N, and NC4N are expectedto result in a reduction in the accompanying chain lenexpansions in the sequence NC4N, HC5N, HC6H. The ob-served trend in the values ofuB0(1Sg

1)2B0(X 2Pu)u ofTable V, namely, HC6H

1.HC5N1.NC4N

1 is consistentwith the distribution of charge in the neutral molecules.

The structural changes for neutral HC6H that occur uponionization to theA 2Pg state are dominated by the reductioin bonding character of the terminal CwC bonds. In thecorresponding molecular orbital for neutral HC5N andNC4N, the CwN group withdraws electron density from thC–C bonds increasing its bonding character. Therefore,izations to theA 2P states of HC6H, HC5N, and NC4N areexpected to result in an increase in chain length expansin the sequence HC6H

1, HC5N1, NC4N

1. This is the ob-served trend for HC5N

1. The value of uB0(1Sg1)

2B0(A 2Pg)u for HC5N1 is larger than that for HC6H

1,consistent with expectations. However, the value for NC4N

1

TABLE V. Comparison of the values ofB0(1Sg1)2B0(X 2Pu) and

B0(1Sg1)2B0(A 2Pg) derived for HC6H

1, HC5N1 and NC4N

1.

B0(1Sg1)2B0(X 2Pu)a B0(1Sg

1)2B0(A 2Pg)a

HC6H1 b 20.00042 0.00038

HC5N1 20.00020 0.00056

NC4N1 20.0001 0.00040

aThe designationsg andu do not apply for HC5N1.

bReference 16.

166.111.132.167. This article is copyrighted as indicated in the abstrac

ei-

e

s

h

n-

ns

does not follow the expected trend. It is smaller than thatHC5N

1 and is even comparable to that for HC6H1.

The odd electron is in molecular orbitals that are linecombinations of the atomic orbitals. The magnitude of tspin–orbit splitting is correlated with the atomic spin–orbcoupling parameters of these atomic orbitals. The valueA09 for HC6H

1 ~'231 cm21! in the X 2Pu state is close tothe atomic spin–orbit coupling parameter of the neutraatom~29.0 cm21!.22 The observed trend in the values ofuA09unamely, HC5N

1.HC6H1 is consistent with the presence o

the N atom with a much larger atomic spin–orbit coupliparameter of 73.3 cm21.22 The value ofDA for HC5N

1

~23.12 cm21! is comparable to that for HC6H1 ~23.00

cm21!, consistent with the isoelectronic relationship. Hoever, the corresponding value for NC4N

1 appears to have ananomalous value~0.53 cm21!. Therefore the value ofA0(A 2Pg) for NC4N

1 is observed to be larger in magnitudthan the expected value~238 cm21! by about 3.5 cm21.

The parameters for the HC5N1 ~X 2P, A 2P! are consis-

tent with the molecular orbital and electron density conserations of the electronic structure based on the isoelectrprinciple. While the parameters for NC4N

1 (X 2Pu) are alsocompatible, those of theA 2Pg state appear to be unusuaFirst, the spectrum of NC4N

1 exhibits a perturbation characteristic of L-type doubling. Second, the value ofDA is sig-nificantly smaller than expected and moreover is positiThird, the value ofDB is smaller than expected. All aspeccan be interpreted in terms of a second-order perturbainvolving the V51/2 component of theA 2Pg state and alow-lying 2Sg

1 state via a spin–orbit induced configuratiointeraction. TheV53/2 component remains unchanged.

The photoelectron spectrum of NC4N shows that the ex-cited 2Pg state, lies energetically near the2Sg

1 and 2Su1

states.23,24 The latter two states are the result of ejectionelectrons from molecular orbitals composed mainly ontwo axial N lone pair orbitals~4sg and 3su!. Of these two

t. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

in

-

tlyse

re

ea

tioc

e

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-

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-

rv

9607J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Electronic spectra of cyanopolyacetylenes cations

D

states, the2Sg1 state has the correct symmetry to be the

teracting state.The L-type doubling observed in theA 2Pg state is a

direct consequence of the2S –2P interaction. The degeneracy of thee and f symmetryL-components in theA 2Pg

state is removed. An unusual aspect of theL-type doublingis that theV53/2 component appears to be significanmore perturbed than theV51/2 component. Figure 3 showthe calculatedDne f for theV53/2 and 1/2 components. Thexperimental values ofDne f are also indicated. At low val-ues ofJ it is seen thatDne f(V51/2) is significantly largerthan that forV53/2. For increasing values ofJ, Dne f(V51/2) begins to decrease and finally the parity orderingverses betweenJ545.5 and 46.5~A in Fig. 3!. For largevalues ofJ, the magnitudes ofDne f are comparable. Thesobservations suggest that spin-uncoupling effects accompthe L-uncoupling.

The increased spin–orbit splitting in theA 2Pg state isproposed to be the result of a spin–orbit induced interacwith the 2Sg

1 state. As a result of the second-order interation, the value ofA0(A 2Pg) for NC4N

1 is observed to belarger than expected by'3.5 cm21. Thus, the interactionpushes theV51/2 component of theA 2Pg state up by 3.5cm21, and therefore pushes the2Sg

1 state down by the samamount. This is consistent with the2Sg

1 state atlower energythan the2Pg ~V51/2! state. The second-order spin–orbcontribution to the rotational constant is given by25,26

B~2!5 (nÞ0

4^ouLun&Eo

02En0 B2. ~1!

UsingDE'41.5 cm21 and^ouLun&51 for P states, Eq.~1! indicates thatB(2)'0.0002 cm21. The ‘‘true’’ value ofB0(A 2Pg) is 0.043 95 cm21, giving a value of DB of0.000 73 cm21 that is comparable to those of HC6H

1 andHC5N

1 ~Table IV!.Finally, the second order spin–orbit contribution to t

A 2Pg state may be used to estimate the location of the2Sg1

state. The coupling element between the2Pg , V51/2 and2Sg

1 states is given by22

FIG. 3. Observed values forDne f(J) in the P branch of theV51/2 com-ponent of theA 2Pg state of NC4N

1. Lines show theL-type doubling cal-culated from the constants of Table IV.~A! Position of parity reversal fortheV51/2 component. Dashed lines indicate the magnitude of the obseFWHM.

ownloaded 13 Sep 2013 to 166.111.132.167. This article is copyrighted as indicated in the abstrac

-

-

ny

n-

^2Pg~V51/2!uHSOu2Sg1u&521/2ASO. ~2!

Using the value ofASO'238 cm21, this gives a matrixelement of about 27 cm21. According to second-order perturbation theory, the shift of the2Pg , V51/2 state is givenby

DE~2Pg ,V51/2!5u^2Pg,1/2uHSOu2Sg

1&u2

@E~2Pg,1/2!2E~2Sg1!#

. ~3!

With the experimental value ofDE(2Pg ,V51/2)'3.5 cm21 and using ^2P1/2uHSOu2Sg

1u&'27 cm21, thisplaces the unobserved2Sg

1 state about 208 cm21 below theA 2Pg, V51/2 state.

Assuming the pure precession hypothesis, theL-typedoubling constantp0 andq0 should be given by

p54AB

EP2EP'0.03, ~4a!

q54B2

EP2ES'3.831025. ~4b!

The value obtained forp is 102 times larger than thevalue observed experimentally. Despite this, the valuetained for q is comparable to the value inferred from thexperimental data. However, the fact that the observed vp0 /q0'10, shows that in this case, the approximationrather inaccurate. Moreover, the fact thatp0 is positive andA0 is negative implies that the dominant perturbing statesabove the2Pg state. Presumably, the states responsiblethe L-type doubling arise from higher vibrational levelsthe 2Sg

1 state.

V. SUMMARY

The electronic absorption spectra of theA 2P←X 2P 000

band of HC5N1~DC5N

1! the A 2Pg←X 2Pu 000 band of

NC4N1 were rotationally resolved and analyzed. The is

electronic principle has shown that theX 2P states ofHC5N

1 and NC4N1 exhibit remarkably consistent behavio

as they are in broad agreement with the molecular orbitalelectron density considerations of the electronic structurethe corresponding neutral molecules. TheA 2P state ofHC5N

1 is also consistent, but theA 2Pg state of NC4N1

deviated from the expectations. Second order spin–osplitting of theA 2Pg state is used to demonstrate the extence of the low-lying2Sg

1 state of NC4N1, at an energy of

about 208 cm21 below theA 2Pg state. This is supported bthe presence ofL-type doubling. Finally, a comparison othe present gas phase data for HC5N

1 and NC4N1 with as-

tronomical measurements shows that no hitherto identiDIBs can be conclusively attributed to the wavelengthstheir origin bands.

ACKNOWLEDGMENTS

This work was supported by the Swiss National ScienFoundation, project No. 20-55285.98. Financial supportthe Ciba–Geigy–Jubila¨ums–Stiftung is gratefully acknowledged.

ed

t. Reuse of AIP content is subject to the terms at: http://jcp.aip.org/about/rights_and_permissions

b,

, J

em

t.

ys.

o-

d

nd

9608 J. Chem. Phys., Vol. 111, No. 21, 1 December 1999 Sinclair, Pfluger, and Maier

D

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