Emission spectra of supersonically cooled haloacetylene cations: X-C

2 emission band systems of rotationally cooled bromo- and iodoacetylene cations, and of their deuterated derivatives, have been obtained by electron i...
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J . Phys. Chem. 1986, 90, 2061-2067

Emission Spectra of Supersonically Cooled Haloacetylene Cations: X-C=C-H( X = Br, I

2061

D)',

Jan Fulara, Dieter Klapstein: Robert Kuhn, and John P. Maier* Institut fur Physikalische Chemie, Universitat Basel, CH-4056 Basel, Switzerland (Received: October 29, 1985)

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The A211 R2II emission band systems of rotationally cooled bromo- and iodoacetylene cations, and of their deuterated derivatives, have been obtained by electron impact excitation of seeded helium supersonic free jets. The low rotational temperature (4 K) leads to a narrowing of the bands and resolution of isotopic splittings so that a vibrational analysis could be made. All the transitions identified belong to the ! I = 3 / 2 component system. As a result of the present data and those objained previcusly from the laser excitation spectra, almost all the fundamental vibrational frequencies of these cations in their X2113/2 and A2113/2states are now known, usually to within A 2 cm-'.

1. Introduction The haloacetylene cations, X--C=C-H+, have often been used as prototype systems in studies of the electronic structure of open-shell polyatomic cations.'" The aim of our studies is to spectroscopically characterize the cations by determining vibrational, rotational, and related constants, and to this end various experimental approaches have been used.7 These have been based on the detection of the emission spectra following electron impact excitation of effusive8 and seeded helium supersonic free jets,9 on laser excitation spectra of cations produced by Penning ionization,1° by a photoelectron-photon coincidence method," and by absorption measurements in Eare-gas_matrices.l2 However, although the A211 X211 emission spectra of the haloacetylene cations, with X = C1, Br, I, were among the first reported at the beginning of our investigations, a vibronic analysis of the band systems was m i ~ s i n g .The ~ complexity of the spectra arises mainly due_ to th_e large change in the C-X distance accompanying the A211-X211 transition (shown diagrammatically from the calculations made for CI-CkC-H+)I3 resulting in extensive vibrational progressions and sequence bands involving the v3 (C-X stretching) totally symmetric mode in both electronic states. Thus even the locations of the origin bands were not known until the laser excitation spectra of these transitions were obtained.14-16 In the latter, due to the means of preparation of the ions,I7 the lowest vibrational level of the X2113/2 spin-orbit componcnt state-is preferentially populated, and the origin band of the A2113/2-X2113/2 subsystem is readily identified. Furthermore, in the higher resolution (0.005-nm) laser excitation spectra the rotational structure of these electronic transitions could clearly be resolved and rotational and related constants have been obtained for the various isotopic species of X-C=t-H(D)+, X = Cl,I4 Br,ISand I.I6 These rotational constants, in addition to that known for diacetylene cation,19 provide the only examples of such data for open-shell cations comprised of more than three atoms. Armed with the information from thejaser excitation spectrum, the emission spectrum of the A211 X211transition of chloroacetylene cation produced rotationally cold in a seeded supersonic free jet could be analyzed.20 In this contribution the corresponding vibrational analysis of the emission spectra of rotationally cold 79,8lBr-C~C-H+ 79,81Br-C~C-D+ I-C-'C-H+, and I-C=C-D+ are presented, and as a result the vibrational frequencies 0-f nearly all the fundamentals are obtained for the X211,1z and A2113/2 cationic states. As IS well documented for molecular species,2I so also for cations the application of the supersonic jet technique results in drastic improvements in the quality of the spectra and in the precision with which vibrational frequencies can be determined.]' The narrowing of the bands in the present case enables the isotope splittings of the 79Br and 81Brspecies to be resolved, this infor-

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Present address: Department of Chemistry, St. Francis Xavier University, Nova Scotia, Canada B2G 2CO.

mation being invaluable in the analysis. Because the vibrational intervals can now be deduced to within f 2 cm-I, as compared to the measurements on effusive beams where the inhomogeneous broadening leads to an uncertainty of f 1 0 cm-]? the progressions and combination series can be followed with more confidence. In addition, sequence bands lying close to the strong progressions are also discernible in the new spectra and in view of the knowledge of the excited-state frequencies deduced from the laser excitation spectra,15J6 they can be assigned. The now almost complete set o,f vibrational frequencies for the haloacetylene cations in their X2113jzstate thus obtained should be useful in further studies of these ions using the various IR laser techniques developed in the recent years,22 in addition to the rotational constants for those

(1) Haink, H. J.; Heilbronner, E.; Hornung, V.; Kloster-Jensen, E. Helu. Chim. Acta 1970,53, 1073. (2) Heilbronner, E.; Muszkat, K. A.; Schaublin, J. Helu. Chim. Acta 1971, 54, 58. (3) Heilbronner, E.; Hornung, V.; Maier, J. P.; Kloster-Jensen, E. J . Am. Chem. SOC.1974, 96, 4253. (4) Allan, M.; Kloster-Jensen, E.; Maier, J. P. J . Chem. SOC.,Faraday Trans. 2 1977, 73, 1406.

( 5 ) Dujardin, G.; Leach, S.; Taieb, G.; Maier, J. P.; Gelbart, W. M. J . Chem. Phys. 1980, 73, 4487. (6) Maier, J. P.; Thommen, F. In "Intramolecular Dynamics"; Jortner, J.,

Pullman, B., Eds.; Reidel: Dordrecht, The Netherlands, 1982; p 241. (7) Maier, J. P. Acc. Chem. Res. 1982, 15, 18. (8) Maier, J. P. Chimia 1980, 34, 219. (9) Carrington, A.; Tuckett, R. P. Chem. Phys. Lett. 1980, 74, 19. Miller, T. A.; Zegarski, B. R.; Sears, T. J.; Bondybey, V. E. J. Phys. Chem. 1980, 84, 3154. Klapstein, D.; Leutwyler, S.; Maier, J. P. Chem. Phys. Lett. 1981, 84, 534. (10) Miller, T. A.; Bondybey, V. E. J . Chim. Phys. Phys.-Chim. Bioi. 1980, 77, 695. Maier, J. P.; Marthaler, 0.;Misev, L.; Thommen, F. In

"Molecular Ions"; Berkowitz, J., Groeneveld, K.-O., Eds.; Plenum: New York, 1983; p 125. (1 1) Maier, J. P.; Thommen, F. In "Gas Phase Ion Chemistry"; Bowers, M. T., Ed.; Academic: New York, 1984; Vol. 3, p 357. (12) Bondybey, V. E.; Miller, T. A,; English, J. H. J. Chem. Phys. 1980, 72, 2193. Leutwyler, S.; Maier, J. P.; Spittel, U. Chem. Phys. Lett. 1983, 96, 645. (13) 353. (14) 3181. (15) 43. (16) 1587. (17)

Botschwina, P.; Sebald, P.; Maier, J. P. Chem. Phys. Lett. 1985, 114, King, M. A.; Maier, J. P.; Ochsner, M. J . Chem. Phys. 1985, 83, Maier, J. P.; Misev, L. J . Chem. SOC.,Faraday Trans. 2 1984, 80, Maier, J. P.; Ochsner, M. J. Chem. SOC.,Faraday Trans. 2 1985,81,

Klapstein, D.; Maier, J. P.; Misev, L. In "Molecular Ions: Spectroscopy, Structure and Chemistry"; Miller, T. A,, Bondybey, V. E., Eds.; North-Holland: New York, 1983; p 175. (18) King, M. A,; Maier, J. P.; Misev, L.; Ochsner, M. Can. J . Phys. 1984,

62, 1437. (19) Callomon, J. H. Can. J. Phys. 1956, 34, 1046. (20) Klapstein, D.; Kuhn, R.; Maier, J. P. Chem. Phys. 1984, 86, 285. (21) Levy, D. H. Annu. Reu. Phys. Chem. 1980, 31, 197.

0022-3654/86/2090-2061$01.50/0 0 1986 American Chemical Society

2062

The Journal of Physical Chemistry, Vol. 90, No. 10, 1986

Fulara et al.

3f

3: I

31 I

3: I

1

I

I

23000

I

I

I

I

I

-

I

20000

20000

1 I

21000

22000

23000

1

21000

22000

I

I

19000 18000 17000 G(cm'') Figure 1. The ,&211,i2 %2113,2emission spectrum of bromoacetylene cation obtained by ~200-eVelectron impact excitation on a seeded helium supersonic free jet. The optical resolution was 0.07 nm (fwhm). Some of the vibrational assignments are indicated and the consecutively numbered vertical markers, intermittently labeled with the circled numbers, reference the bands listed in Table I. Atomic lines (He, Br) are

marked with a dot. interested in the microwave spectra of ions.23

2. Experimental Section Samples of the respective haloacetylene at vapor pressures of ~ 2 mbar 5 were premixed with helium gas (=1.0 bar) and then expanded through a 70-pm nozzle into the ionization region of the electron beam apparatus.I7 The generated supersonic free jet was struck by a collimated =200-eV electron beam, current 3-6 mA, about 5 mm downstream of the nozzle. Any emission thus excited was observed by a lens system coupled to anf/9.5 1.26-m monochromator and single-photon-counting electronics. The spectra were recorded on line with an LSI 11/03 microcomputer which also steered the wavelength drive of the monochromator. The calibration of the wavelength scale was automatically provided by the numerous helium and halogen emission lines excited concomitantly. For the higher resolution recordings of selected bands, however, a neon-filled thorium hollow cathode lamp was employed. The samples of the bromo- and iodoacetylenes, and their deuterated derivatives were synthesized as described in the literature;' their purity was monitored by mass and photoelectron spectroscopy. 3. Results and Discussion 3.1. General Remarks. The electronic structure of the monohaloacetylenes was first discussed in relation to their photoelectron spectra.' Thus in terms of a molecular orbital description, the cationic ground state is formed by removal of an electron from an orbital which is bonding in the C-C but antibonding in the C-X region. In the case of the first excited state of the cation, (22) Gudeman, C. S.; Saykally, R. J. Annu. Rev. Phys. Chem. 1984, 35, 387. ( 2 3 ) Woods, R. C. J . Mol. Strucr. 1983, 97, 195.

1

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19000 Figure 2. The A2IIsI2

I

I

I

18000

I

17000

I

C(cm-')

g21'13/2 emission band system of supersonically cooled deuteriobromacetylenecation recorded with 0.07-nm resolution. Further details as for Figure 1.

A211, the electron stems from a molecular orbital bonding throughout the A network. The main consequence of thisiifference is that the C-X distance is about 0.2 A shorter in the XzII compared to that in the A211 state. These changes have been deduced from Franck-Condon factor calculations for the photoelectron bands2 and from the recently obtained rotational constants for the bromo-L8 and i_odoacetylene16cations. The rotational analysis of the AZII XzII electronic transition of these cations also show that the linearity of the species is maintained in these two states. Each of the W Z I I and AZIIelectronic states is split by spin-orbit coupling, giving rise to the R = 'i2 and R = substates. Whereas the spin-orbit spjitting in the XzII state is smaller (0.13 f 0.02 eV) than in the AZIIstate (0.20 f 0.02 eV) for bromoacetylene cation, the opposite is the case for iodoacetylene cation: Le., (0.40 f 0.02 eV) and (0.25 f 0.02 eV), res~ectively.~.~ This then gives the relative energies of the AZIIy2 X211,iz and A2IIIj2 22111/2 subsystems. However, as the vibrational analysis of the emission spectra show (sections 3.2, 3.3), bands belonging only to the R = 3 / 2 systems are identified. The assignment of the bands uses the numbering of the five fundamental vibrational modes as follows. There are three totally vl to v3 modes, which can be described as v(C-H), symmetric ,)'a( v(C-C), and v(C-X), respectively, and two doubly degenerate (a)bonding modes, v4, b(C=C-H), and v5, S(C=C-X). The frequencies of their fundamentals in the XI Z+ molecular ground statez4 are listed in Tables I11 and VI. 3.2. Bromoacetylene Cations. The main portions of the A211 X211emission spectra of rotationally cooled (T,, = 5 K) bromoand deuteriobromoacetylene cations are shown in Figures 1 and 2. The vibrationally rich band system extends from 430 to 600 nm and on most of the bands isotope splittings due to the 79Br

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(24) Hunt, G. R.; Wilson, M. K. J . Chem. Phys. 1961,34, 1301. Brown, J. K.; Taylor, J. K. Proc. Chem. SOC.1961, 13. Carpenter, J. F.; Rimmer, D.F.;Whiffen, D. H. J . Chem. SOC.,Faraday Trans. 2 1972, 68, 1914. Anttila, R.; Kauppinen, J.; Huhanautti, M.; Karkkhinen, T. J. Mol. Specrrosc. 1977, 64, 460.

Emission Spectra of Haloacetylene Cations

The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 2063

TABLE I: Maxima of the Prominent, or Assigned, Bands in the A211yz Cation" label 1

vvacICm-'

t 1 22711 227141 23007 23 002 22 953 22 948

2 3 4

22 522 22518 5 22 499 504 22458 22 420 22 327 22 223 22 03 1

6 7 8 9 10 11

22 020 22017 12

t

t

t

3;x

263; 3;x

gzlIa/2 Emission Spectrum of Supersonically Cooled Bromoacetylene label 47 48 49 50 51 52

3: 53 54 3:x 2i3$ 30x

55 56

3;

57

vvac/cm-'

assignment

19913 19878 19796 19 700 197031

t

19694 19691 19 605 19 602) 19 588 19 509 19425 194151 19 334 l 9 210 19 206

3;5;; 3;5;

21 936 21 845 21 814 21 731 21 538 21 532

13 14 15 16 17 18

assignment

-

t

19 101

3:x

58

2p3:

59

30 3!x

60

3;s;; 3;5:; 3;5;

61

3jx 31

62 63 64

t

19034 19 030 19016 19011t 18 920

21 441 21 398 21 357 21 346 21 156 21 054 21 043 20 968 20 949 20 907 20 865 20861

19 20 21 22 23 24 25 26 27 28 29 30

20 772 2o 779 20755 20681 2o 20 656 20581 20551

31 32 33 34 35 36

20 480 2o 490 20 378 20371

37 38 39

2o 285 20 28 1 20 254 20217 20 189 20 160 20 092 20 074 20 006

40 41 42 43 44 45 46

X

3459; 3;5;

t

65 66

305;

67

31x 3; 3152. 32 I I 0, 151

68

3;x

I

I

t l 8 263 18 258 t l18 8 541 536 18 430

177t

l 8 172 18 69 70 71 72

t

17951 17 945 17 870 17 8641 17763 17688 17 590

73

t

18 848 18 749 18620

000 3;5;

3yx 31 375;; 315;

17414t 17408

74

17 274 75 17 189 76 77 78

3:; 3:x 3:s;

79 80

5;

81

l17 7 101 lost 17 004

16615t 16 607 16 530

16432 '6440)

16330t 16321

'The bracketed bands correspond to the individual isotopic species and/or to multiplet vibronic structure. The labels refer to the numbering in Figure I . All values f l cm-'.

and s'Br constituents are discernible. The wavenumbers of the maxima of the stronger or assigned bands are collected in Tables I and 11. The origin of the A2113/2 W2113/2 spin-orbit subsystem is located at 20551 i 1 cm-' for the bromoacetylene cation (the isotope splitting being less than 1 cm-I) and is red-shifted by 5 cm-' on deuteration. While this transition is observed only very weakly (cf. Figures 1 and 2), its identification follows from the vibrational analy_is, apart from the fact that it is unambiguously identified in the A211312 X2n3/2laser excitation spectrum where this band is the most intense of the system.I5 Actually, from the

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-

rotationally resolved excitation spectra the origins of the individual isotopic species have been determined.ls The strongest bands in the emission spectra of the bromoacetylene cations are associated with the excitation of the vi(C-Br str) mode (which shows the largest isotope effect) in the X2113/2 and A211?/2states in the form of progressions 3;. Some of the band assignments are marked in Figures 1 and 2. The inferred fundamental frequencies ( f 2 cm-') of the v, mode are vJN = 673, v3' = 492 cm-' and v3" = 658, v3/ = 484 cm-' for bromo- and deuteriobromoacetylene cations, respectively. Transitions involving the Lxcitation of the vl", UT, and vy (in

2064

The Journal of Physical Chemistry, Vol. 90, No. 10, 1986

TABLE 11: Maxima of the Prominent, or Assigned, Bands in the Deuteriobromoacetylene Cationa label 1 2

uvaclcm-' 23 091

3

22613t 22610

2;3:

22 458

3:

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

22431 22 421 22 131

t

21 923 21 804 21 777 21 647 21 509 21 480 21 465 21 449

t

36 37 38 39 40 41 42 43 44 45 46 47

:5; 3;x 3;s:; 3;5;

label 53

54

v,.,lcm-' 19638 19515 19510 19 506 19503

assignment 33;; 3:5;

55 19471 19462 56 57 58 59

t l 9 241 t 19238 19214 l19 9 381 379 19343

19 2091 60 19 164 61 62 63 64 65

:iX

66

34

67

305; 365;; x 3: 3;x 315;

19 070 19 028 19 O25} 19 020 18989 189871 18961 18906 18 9021

18810t 18680

68 69 70

18587 18 585 18548 18443

3:x

t

503t

71 72

0: 3;5;

18 374 18 344 18 334) 18330

73 74

4791

2o 475 20 20 375 20361 20 346 20 323 20 299 20 245 20 203 20 161

026t 17943t 18 020

31 3yx 375:

75 76 77

17937 17901 17 797

78 3: 3;x

17681 79

17420t 372t 17416

20 144 20 108 20 030 l 9 888 19 891

48 49 50 51

Emission Spectrum of Supersonically Cooled

jt2n3/2

18 807

2o 20 494 35

2b3:

!X : 2h3;

20 720 20 647 20618 20 591 2o 587 20 546

3;x 3; 345; 30x 3205;

21 985 21 959 21 948

21 401 397 21 331 21 297 21 153 21 030 20 996 20 974 20 855 20813 20783

-

340x

22 903 5

assignment 2b3: 3;

22 930 4

A2113,z

Fulara et al.

19856 19 829 19722

80

I

3;5; 5; 3?

315; 375; 3:

19698

l17 7 364 81 82 83 84

3;x 19 669

1

16770 l6 85 86 87

52

l 7 293 17285 17256 17 209

7231

l16716 6 16631 16 567 16562 16551

"The labels refer to Figure 2 and bands of the isotopic species or those consisting of multiplets are bracketed. All values f l cm-'

double quanta) modes are also identified. These occur as short progressions and/or mainly as combination series with the u3 mode both in the ground and excited electronic states. In the excited

state, excitation of the u{ mode is also apparent and again combination series built around this are identified. For comparison, in the laser excitation spectrum of this transition, the excitation

The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 2069

Emission Spectra of Haloacetylene Cations TABLE III: Vibrational Frequencies (cm-') of Bromo- and Deuteriobromoacetylene Cations in Their Ground and First Excited Electronic States" ?r

U+

VI

species H-CEC-Br H-C=C-Br+

state

v5

Y4

(YC-B~) (~CCH.D)

xlZ* 3325 X2113/2 3280

2085 1931

618 673

A2n3,2

2051* 1950 1866

492 606 658

618 618 lob 629* 480 544f

1939*

484

488*

x12+ 2600 X2113,2 2482

D-C4-Br D-C=C-Br+

Y3

y2

(YCH.D)

*

lob A2113/2

2548*

(~ccB~) 295 273 1oc 207* 283 258 f 10c 200*

*

"The frequencies given are for the 79Br isotopic species. The values marked with an asterisk are taken from the laser excitation spectrals and the molecular ground-state values from ref 24. All values f 2 em-l except as indicated otherwise. bCalculated assuming X = 3b41 and using the value of Y.,' from ref 15 (cf. section 3.2). 'Calculated as '/2(2ysll).

of the v2' and u3' stretching as well as of the vql and v5' bending modes is indicated.I5 All the inferred vibrational frequencies are

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TABLE I V Maxima of the Prominent, or Assigned, Bands in the AfI13/2 Cations" label 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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

v,,clcm-'

assignment

19030 18979 18957 18637 18 584

t

l18 8 573 567 18 403 18 325 18 242 18 185 18 172 18 044 18 008 17 983 17943 17 782 17 773 17609

592t

l17 7 588 17 539 17435 17419 17 375 17 350 17 325 17311 17 207

197t

l 7 190 17 17 138 17036 17007 16955 16917 16797

16788t

16714 16 737 16 633

166135

16 608 16 568 16 556 16464 16 375

3;5; 3: (53 3;5; 30 3;5i 3150

summarized in Table I11 and as one can see, with the exception of the v 1 value in the excited state for bromoacetylene cation, all the frequencies are known. In addition to the assignments mentioned above and as given in Tables I and 11, in each of the emission spectra of the two bromoacetylene cations there is a combination series involving the v3 modes in the lower and upper states together with a mode designated as X. The apparent origins lie 503 and 428 cm-I to the blue of the 0; bands for the bromo- and deuteriobromoacetylene cations, respectively. The intensity and isotope pattern of the 3 i X series built upon this are very similar to the 3; progression. The 31X, 3;X, and 3:X bands are the strongest ones and the isotope splittings for 3hX, 3iX, 3iX, and 3:X are 1.1, 2.5, 3.6 and 4.9 cm-I, to be compared with 0.9, 2.2, 3.3, and 4.3 cm-I for the 3;, 3i, 3:, and 3:transitions. It can be excluded that these transitions belong to the 0 = 1 / 2 subsystem Le., X 5 0.; Although the origin 0; would lie in the right energy region according to the photoelectron data,'s4 the isotope shift of 80 cm-I to the red on deuteration is inconsistent with the 5-cm-I shift for the 0; (a = 3/2) origin. Thus in the bands emission spectrum of the bromoacetylene cations the 0 = are too weak to be identified. A similar situation prevails in the X2n, transition of bromoemission spectrum of the BZII,

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%211s/2Emission Spectrum of Supersonically Cooled Iodoacetylene label 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

vVac/cm-I 16 346 16 330 16292 16226 l6 16 204 16 153 16 124 16097 16061 16 028 15 976 15 950 15 746 15718 15 654

2061

assignment 3750, (402) 30, 3;5; (3149 3: 3025: 30250, (3?43 3: 3;5;

3:5; 3i5: 32

O; 51 31 3;5;

375; 3; 33; 3;5: 3 315:

59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85

15626 15551 15510 15490 15399 15 165 15 154 15 107 15083 l15067 5O7ll 14998 14 827 14700 14658 14610 14589 14524 14510 14 424 14117 14033 14017 13946 13 855 13 539 13448 13372 13287

362y5; 3:

3;51

33%

215; 34 3:5; 273: 3:57

3;50 273!50, 3; 2'3' 3050 2!3!5! 2:3! ( 1? 3 3

3050 2'3'5; a13 2134

'The labels refer to Figure 3. Bracketed values correspond to multiplet structure of the bands. Assignments in parentheses are tentative. All values * 1 cm-'.

2066 The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 TABLE V Maxima of the Prominent, or Assigned, Bands in the A2113,2 Deuterioiodoacetvlene Cationa label 1

2 3 4

5 6 7 8 9 10 I1 12 13 14 15

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

v,,c/cm-'

18997 18 956 18613 18 569 18 554 18391 18 224 18 181 18 168 18051 18 008 17985 17949 17899 17 786 17780 17664 17619 17 599 17 562 17449 17 422 17 389 17 354 17 276 17 225

41

~,,c/Cm-l

33: 3: " 33; 3; 3 3j 3:

42 43

16267

5: 3: 331 3i5! 5; 3: 33; 3: 39;

166491

37 38 39 40

label

375; 3; 3151

17211 17 166 17059 17033 16993 16934 16826 16716 16 665 16 644

33:

16604 16 545 16502 16 426

3; 3:s;

3821

l6 16 378

W * I I S , ~ Emission Spectrum of Supersonically Cooled

assignment

00,

17219t

-

Fulara et al.

33;

assignment

16258t 16251 16210 16 168 16 108 16 079 16036 15991 15823 15812 15710

44 45 46 47 48 49 50 51 52 53

15690t 15 686 15594 15 552 15472 15435 15 390 15244 15 198 15 154 15 122 15 079 15045 14913 14771 14745 14681 14630 14 598 14559 14519 14232 14 199 14 121 14 064 14041 13 963 13 928 I3 628 13565 13503 13456 13410 13 339

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85

Multiplet structured bands are bracketed. All values f 1 cm-'

cyanide cation where also only bands of the fl = 3 / 2 component are apparent.25 The remaining possibility is that the v4 degenerate bending mode is involved. That X = 4; can be ruled out because the resulting difference in the vibrational frequency of the vql fundamental on deuteration would be only 80 cm-I, whereas the difference of the molecular frequencies is 138 cm-' (cf. Table 111). In addition, the u4/ values deduced (though tentatively) from the laser excitation spectra of the bromo- and deuteriobromoacetylene cations are 629 and 488 cm-I, respecti~ely.'~ Alternatively if X is assigned as 3;4:, the origin of the series lies near the 0; band and is apparently rather weak, as are the 3k4; and 3 i 4 ; combination bands. If this is correct, then the uq/l frequencies are evaluated to be 6 18 and 544 cm-' on the basis of this data and the vql frequencies. This then implies that whereas in the case of bromoacetylene cation the [v value is the same as in the molecular ground state, there i_s =lo% increase in the frequency for the deutero species in the XzII state. 3.3. Iodoacetylene Cations. The A211 g211emission band systems of rotationally cooled iodo- and deuterioiodoacetylene cations are observed in the 525-750-nm region (Figures 3 and 4). The spectra consist of many bands and in Tables IV and V

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(25) Fulara, J.; Klapstein, D.; Kuhn, R.;Maier, J. P. J. Phys. Chem. 1985, 89, 4213.

the frequencies of the stronger or identified transitions are given. As is the case with the emission of bromoacetylene cations, also with the iod-mcetylene cations only transitions associated with the A2n3,2 X2113/2subsystem are assigned. Bands belonging to the Q = ' I zcomponents are very weak or not present under such experimental conditions. The origin band of the fl = ' I 2system is again quite weag due to, the considerable geometry change accompanying the AZII XzII transition, and is found at 17 375 f 1 cm-'. It undergoes a blue shift of 14-cm-I on {euteration. The Oi band is the most intense one in the A2113/2 X2113/2 laser excitation spectra of these two iodoacetylene cations and from the rotational analysesI6 the proper origins of the bands have been determined (17 373.94 (3) and 17 388.07 (3) cm-I) rather than the R head maxima to which the values listed in Tables IV and V refer to. The emission spectra of the isotopically FiUbstitUted iodoacetylene cations have a very similar pattern (Figures 3 and 4) and the Crongest bands involve the excitation of the u3 vibration in the X2n and A211 states. The determined fundamental frequencies (*2 cm-I) are 578 (563) and v{ = 407 (397) cm-'; the values in parentheses are those of the deuterated species. The excitedstate values are in agreement with the laser excitation data; the latter also provide the u i and usf frequencies.16 In the emission spectrum, however, the v i mode is not active and the v5' mode is only observed in the combination series with the v3' mode. -+

-

-

The Journal of Physical Chemistry, Vol. 90, No. 10, 1986 2067

Emission Spectra of Haloacetylene Cations 3: I

3: I

3: I

31 I

31 I 3; I

3: I

3% I

3:

3:

1

I

31,

I

I

I

jlJ.

IJ1'

38 3: I

I

3:

I

19000

17000

18000

I

I

38

J

19000

31

38 I

I

18000

38 I

I

31, I

I

3P

I

I

17000

38

I

39 I

16000 Figure 3. The A2113/2

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15000 14000 V(cm-') %2113/2 emission band system of iodoacetylene

cation in a supersonic free jet recorded with 0.07-nm band-pass. Atomic lines (He, I) are marked with a dot. Further details as in Figure 1 legend.

TABLE VI: Vibrational Frequencies (cm-I) of Iodo- and Deuterioiodoacetylene Cations in Their Ground and First Excited Electronic States' U+

71

VI

v2

v3

state

(VC-H,D)

(YC-I)

x'Z*

3360 3258

(vex) 2060 1805 f 1Ob 1822* 1742* 1Ob 1792*

species H-CEC-I H-CEC--I*

X2113/2

D-CEC-1'

R2113/2

A2n3/2

A2n3/2

2618

v4

v5

407 563

( ~ c H . o )( ~ C C I ) 630 267 542 f 237 1o c 612* 212d 223

397

480*

490 578

225 f 1o e

The frequencies taken from the laser excitation spectral6 are denoted with an asterisk, and the molecular ground-state values are taken from ref 24. All values f 2 cm-' except as indicated otherwise. bEvaluated from 2y3: and 2?3:5: combination bands (cf. section 3.3). CCalculatedas 1/2(2u,,"). dEvaluated from the 5 ; sequence transition, whereas the value of 234 cm-' given in ref 16 was estimated by using 1/2(2v5') which is affected by Fermi interactions. e Calculated from 2u5' as in ref 16.

In the ground state of the iodoacetylene cations, the vgll mode is excited in double quanta in series with the v3" mode, Le., 5;30,, and as sequences, 513$, and in addition in vibrationally forbidden transitions, 573;. These transitions become vibronically allowed due to Renner-Teller inieractions. A related observation was noted in the B2n,A28+ X211 emission spectra of the halocyanide

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16000 Figure 4. The AZI13/2

15000 14000 S(cm-') %2113,zemission band system of supersonically

cooled deuterioiodoacetylene cation (0.07 nm). Further details as for Figure 1.

cations, where in the bromo species the degenerate mode is excited mainly in double quanta (as is the case with bromoacetylene cation), the result of which is a vibrationally allowed u+ component, but in the iodo species relatively strong single quantum excitation of these modes is also favored.25 In the spectra of the iodoacetylene cations (Figures 3 and 4) the bands involving the excitation of the v5 mode exhibit a doublet, or multiplet, structure owing to the vibronic interactions. Besides the v3" and us'' frequencies, the values of the Y," and v2/1 modes have also been determined from the spectra. The v2" frequencies are 1805 (1 742) cm-' for iodo- (deuterioiodo-) acetylene cations but their uncertainty is estimated as f10 cm-' because they could only be evaluated from the 2y3: or 293:5: series, the 2: bands being too weak to be observed. The Y; values obtained from the two series differ by up to 10 cm-', due to the differences in anharmonic constants and Fermi interactions. All the presently determined fundamental frequencies are summarized in Table VI and they are supplemented by the values from the laser excitation spectra.

Acknowledgment. This work is part of project No. 2.429-0.84 of the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung. Ciba-Geigy SA, Sandoz SA, and F. Hoffmann-La Roche & Cie SA, Basel, are also thanked for financial support. Registry No. HC=CBr+, 34500-61-3; DC=CBr+, 89573-79-5; HCGCI', 34500-62-4; DC=CI+, 100692-33-9.