The Asymmetric N-O Stretch Fundamental Band of Nitromethane: Intramolecular Vibrational Relaxation in the Lowest Internal Rotor State DAVID S. PERRY, LOU DEGLIUMBERTO, University of Akron, Akron, OH 44325 WENDY HEISERMAN, REBECCA COOK, HOWARD D. METTEE, Youngstown State University, Youngstown, OH 44555 ROBERT L. SAMS, Pacific Northwest National Laboratory, Richland, WA 99352 GEORG O. SORENSEN, University of Copenhagen, DK-2100, Copenhagen, Denmark. Work supported by: Department of Energy Pacific Northwestern Laboratories

Slit-jet FTIR Spectroscopy from Bruker FTIR coupling optics to large roots pump system CW slit nozzle Rotational temperature is approximately 15K Spectral resolution is 0.0025cm-1 Bob Sams Environmental Molecular Science Laboratory Pacific Northwest National Laboratory Richland, WA

Background Slightly-oblate asymmetric rotor ( =0.25) G12 symmetry Low 6-fold barrier to internal rotation b-type band centered at 1583cm-1 Selection rules J=0,+1  Ka=+1,3  Kc=+1,3  m=0 Nitromethane is a near oblate asymmetric rotor with an asymmetry parameter is 0.25. It has G12 symmetry in permutation-inversion theory. Even though nitromethane is more oblate than prolate, we chose the prolate axis as our frame of reference since this is also the axis of internal rotation and any fx of the torsion on the rotational structure wil be more clearly seen here. The NO asymmetric stretch is a b-type band with the following selection rules….Since it is more oblate than prolate dKa+-3 is an allowed transion. M is the torsional angular momentum quantum number.

16O spin statistics: Only Ka+ m + v6 = even allowed Assigned Transitions Due to the symmetry of the rotation, vibration, and torsional states only every other prolate K is allowed. 16O spin statistics: Only Ka+ m + v6 = even allowed

Assignments 1638 transitions have been assigned using combinational differences 1185 transitions for the m =0 state 256 transitions for the m =1 state 72 transitions for the m =2 state 125 transitions for the m =3 state

Clumps Most transitions in the lowest torsional state Clump Center From perturbed graphs -1 Most transitions in the lowest torsional state are split into clumps of four or five lines. No splitting has been found in the higher torsional states.

Clumps Ka´=3 Kc´=3 Nitromethane is asymmetric From Kcplots of clumps.pxp Nitromethane is asymmetric  follow the interactions as a function of J, Ka, and Kc.

Upper State Energies ´ ´

Ka´=1 Ka´=3 Reinsert pics in different format Ka´=5 Ka´=7

Deconvolution ´ Two step Lehman/Lawrence-Knight deconvolution The decolvolition was carried out in two steps, first the smaller splittings were decolvoled, then the centers of gravity of these were used to deconvolve the bigger splittings. (Reinsert pics in different format) ´ Two step Lehman/Lawrence-Knight deconvolution

Matrix Elements Matrix Element / cm-1 Ka´ Kc´

Matrix Elements J´=8 The matrix elements decrease with Ka From big matrix2.pxp (Reinsert pics in different format) The matrix elements decrease with Ka and increase with Kc

Normal Modes 1 A1 CH Symmetric Stretch 2972 cm-1 2 CN Stretch 3 NO Symmetric Stretch 1397 cm-1 4 NO Symmetric Bend 658 cm-1 5 CH Symmetric Bend 1380 cm-1 6 A1 NO Asymmetric Stretch 1583 cm-1 7 NO2 Wag (in plane) 477 cm-1 8 A2 NO2 Wag (out of plane) 605 cm-1 9 E CH Asymmetric Stretch 3065 cm-1 10 CH3 Rock 1100 cm-1 11 CH Asymmetric Bend 1482 cm-1 States with correct symmetry in torsionally excited states

Bright State Dark States A1´+A2´+E´ A1´+A2´ c-axis Coriolis coupling C-type coriolis coupling will couple states with A2’ to states with A1’ symmerty

Conclusions Most transitions in the lowest torsional state are split into clumps Only the m=0 states are perturbed  perturbing state(s) are torsionally exited. The coupling matrix elements decrease with Ka increase with Kc  c-axis Coriolis coupling with XX XX bands.