The Asymmetric NO Stretch Fundamental Band of Nitromethane

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, 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 CW slit nozzle • from Bruker FTIR • coupling optics Rotational temperature is approximately 15 K Spectral resolution is 0. 0025 cm-1 to large roots pump system Bob Sams Environmental Molecular Science Laboratory Pacific Northwest National Laboratory Richland, WA

Background • • • Slightly-oblate asymmetric rotor ( =0. 25) G 12 symmetry Low 6 -fold barrier to internal rotation b-type band centered at 1583 cm-1 Selection rules J=0, +1 Ka=+1, 3 Kc=+1, 3 m=0

Assigned Transitions 16 O spin statistics: Only Ka+ m + v 6 = 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 Clump Center -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 Nitromethane is asymmetric follow the interactions as a function of J, Ka, and Kc. Kc´=3

Upper State Energies ´ ´

Ka´=1 Ka´=3 Ka´=5 Ka´=7

Deconvolution ´ Two step Lehman/Lawrence-Knight deconvolution

Matrix Element / cm-1 Matrix Elements Ka´ K c´

Matrix Elements J´=8 The matrix elements decrease with Ka and increase with Kc

Normal Modes 1 A 1 CH Symmetric Stretch 2972 cm-1 2 A 1 CN Stretch 918 cm-1 3 A 1 NO Symmetric Stretch 1397 cm-1 4 A 1 NO Symmetric Bend 658 cm-1 5 A 1 CH Symmetric Bend 1380 cm-1 6 A 1 NO Asymmetric Stretch 1583 cm-1 7 A 1 NO 2 Wag (in plane) 477 cm-1 8 A 2 NO 2 Wag (out of plane) 605 cm-1 9 E CH Asymmetric Stretch 3065 cm-1 10 E CH 3 Rock 1100 cm-1 11 E CH Asymmetric Bend 1482 cm-1

Bright State Dark States c-axis Coriolis coupling A 1´+A 2´+E´ A 1´+A 2´

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.