Laser spectroscopy of a halocarbocation: CH 2 I + Chong Tao, Calvin Mukarakate, and Scott A. Reid Department of Chemistry, Marquette University 61 st International Symposium on Molecular Spectroscopy

Background Using a pulsed discharge through a mixture of CH 2 I 2 in He, we observed a series of (partially) rotationally resolved bands in fluorescence excitation spectra with band origin around cm -1

Emission spectroscopy Emission spectra show two distinct progressions, with additional weaker features The progressions involve vibrational modes with frequencies near 750 and 1400 cm -1 We fit the derived term energies to a 2-mode Dunham expansion and compared with calculated ground state vibrational frequencies for a variety of candidate molecules

Emission spectroscopy: Fit results ParameterValue 11 1398(4) 22 755(1) x (13) x (7) x (2) Fit statistics: 11 lines,  = 0.5 cm -1

CandidateCalculated vibrational frequenciesRef. CH 2 I CH 2 I CH 2 I CHI CHI CHI Exp B3LYP/6-311G** (this work) 2 P. Marshall, G. N. Srinivas, M. Schwartz, J. Phys. Chem. A 2005, 109, M. Schwartz, P. Marshall, J. Phys. Chem. A 1999, 103, Calculated or literature frequencies for candidate molecules

Experiments with CD 2 I 2 We subsequently carried out experiments with CD 2 I 2 as precursor The peaks in both excitation and emission spectra shifted upon deuteration, indicating that the molecule contained at least one H atom We compared the observed shifts to calculated values at the B3LYP/6-311G** level

Experiment and theory: CH 2 I + SpeciesFrequencies CH 2 I + (calc.) Experiment CD 2 I + (calc.) Experiment Ratio (d 2 /h 2 ) Ratio (Exp.)0.76(1)0.92(1)

Experiment and theory: CH 2 I 2 + SpeciesFrequencies CH 2 I 2 + (calc.) Experiment CD 2 I 2 + (calc.) Experiment Ratio (d 2 /h 2 ) Ratio (Exp.)0.76(1)0.92(1)

Summary: isotope shifts The deuterium isotope shifts are consistent with assignment to CH 2 I + On the basis of these results, we exclude CH 2 I 2 + as a possible carrier Mode assignments (CH 2 I + ) : 2 =1398 cm -1 CH 2 scissor 3 =755 cm -1 C-I stretch

The C-I stretching mode The mode at 755 cm -1 is the C-I stretch, consistent with the small isotope shift observed upon deuteration Typically, C-I stretching modes are in the range cm -1 The larger frequency indicates some double bond character, consistent with the following resonance structures: The calculated (B3LYP/6-311G**) bond length is significantly shorter than typical C-I single bonds (2.16 Å) C I + + R C-H =1.087 Å, R C-I =1.963 Å  HCH =120.3 ,  HCI =119.9 

What is the excited state? The ground electronic state of CH 2 I + is 1 A 1 We would expect a singlet to singlet transition; however, CIS/6-311G** calculations show that the lowest excited singlet states all lie in the UV region A triplet A 1 state is predicted to lie in the visible region; this transition is spin-forbidden but allowed by spin-orbit interaction To determine the symmetry of the excited state, we modeled the rotational contour in the excitation spectra using the calculated ground state rotational constants, varying the upper state constants. The electron spin in the excited state was neglected, and the 3:1 weighting for ortho:para rotational states due to the H nuclear spin statistics was included

Simulation of origin band The simulation shows an a-type contour. In the C 2v point group,  a =  z has A 1 symmetry. Since the ground state is A 1, the upper state is also A 1. a b c z y x

Back to the emission spectra! Two other (weaker) progressions are observed starting at ~ 1747 and 1918 cm -1 These do not correspond to any calculated fundamental, and thus must be overtone or combination bands Only even overtones or appropriate combinations of non-totally symmetric vibrations are allowed

Mode assignments The two fundamental vibrations ( 2,  3 ) that we observe are of a 1 symmetry For non-totally symmetric modes, only even overtones or appropriate combinations can be observed. We assign the 1747 cm -1 band to 2 6 and the 1918 cm -1 band to 2 4. ModeFreq (calc) Symmetry 13117a1a a1a1 3753a1a b1b b2b2 6899b2b2

Dunham fit to 4 vibrational modes ParameterCH 2 I + Theory (b) CD 2 I + TheoryRatio (d 2 /h 2 )Theory 2 (21) (36) (1)0.76 3 (8) (21) (1)0.92 4 (6) (13) (1)0.78 6 (7) (18) (1)0.75 x (6)…-10.6(11)……… x (4)…-3.8(6)……… x (5)…-4.1(7)……… x (1)…-2.1(3)……… x (2)…-1.3(6)……… x (6)…-3.7(16)……… x (a) …-4.9 (a) ……… x (a) …-4.3 (a) ……… (a) Estimated and fixed in the fit. (b) Calculated at B3LYP/6-311G** level. Fit statistics: CH 2 I +, 20 levels,  = 0.7 cm -1 ; CD 2 I +, 21 levels,  = 1.3 cm -1.

Comparison with condensed phase results Many studies have examined CH 2 I 2 photolysis in condensed phases (see., e.g., work of D. L. Phillips, University of Hong Kong) The major species formed is iso-CH 2 I--I, a weakly bound isomer which arises from geminate recombination in the solvent cage The charge distribution of the CH 2 I moiety in this isomer is similar to that of CH 2 I +, and the vibrational frequencies are similar

Summary We have observed the first gas-phase electronic spectrum of a halocarbocation Comparison of measured vibrational frequencies with calculated values supports assignment to CH 2 I + Excitation spectra show an a -type transition, consistent with an excited state of A 1 symmetry Calculations predict a triplet A 1 state lying in this region, intensity is gained by spin-orbit coupling Calculations also predict a very strong (  = 0.3) singlet- singlet transition lying in the near UV. Measuring the spectrum of CHDI + will be helpful in confirming our results and gaining additional insights

Acknowledgements Useful discussions: Dennis Clouthier James Kincaid Rajendra Rathore Funding: NSF ACS/PRF