Spectroscopy of the $\tilde A$Ã state of NO–alkane complexes (alkane = methane, ethane, propane, and n-butane)

Abstract

We have recorded (1+1) resonance-enhanced multiphoton ionization spectra of complexes formed between NO and the alkanes: CH(4), C(2)H(6), C(3)H(8), and n-C(4)H(10). The spectra correspond to the à ← X̃ transition, which is a NO-localized 3s ← 2pπ* transition. In line with previous work, the spectrum for NO-CH(4) has well-defined structure, but this is only partially resolved for the other complexes. The spectra recorded in the NO(+)-alkane mass channels all show a slowly rising onset, followed by a sharp offset, which is associated with dissociation of NO-alkane, from which binding energies in the X̃ and à states are deduced. Beyond this sharp offset, there is a further rise in signal, which is attributed to fragmentation of higher complexes, NO-(alkane)(n). Analysis of these features allows binding energies for (NO-alkane)···alkane to be estimated, and these suggest that in the NO-(alkane)(2) complexes, the second alkane molecule is bound to the first, rather than to NO. Calculated structures for the 1:1 complexes are reported, as well as binding energies.

Infrared Activated Signatures and Jahn–Teller Dynamics of NO–CH4Collision Complexes

“…The asymmetric CH stretch with A ′ symmetry is characterized by a stretching of the facial −CH 3 bonds and a concomitant compression of the HC- bond. Using the ground state intermolecular bond dissociation energy ( D 0 ∼ 100 cm –1 ) determined previously , and the asymmetric CH stretch frequency (ν CH ) of NO–CH 4 from the IR experiments presented here (3045 cm –1 ), the available energy can be determined, E avl = 2945 cm –1 . The E avl is partitioned to the translational and internal energies of the NO + CH 4 products.…”

“…Previous studies , have determined the intermolecular bond dissociation energy ( D 0 ) of the NO–CH 4 complex to be D 0 ∼ 100 cm –1 . Using the following equation E avl = E h ν − D 0 = TKER + E int ( NO ) + E int ( C H 4 ) along with the IR photon energy E h ν = 3045 cm –1 and D 0 from the literature, the available energy E avl is distributed to relative translation (TKER) and the internal energy of the co-products, E int (NO) and E int (CH 4 ).…”

“…6,8 Experimentally, the REMPI spectrum was concluded to be consistent with an effective C 3v geometry in both the ground and first excited electronic states. 6,8 However, Crespo-Otero et al determined through computational studies that NO−CH 4 is a dynamic Jahn−Teller distorted complex on the ground electronic state (Figure 1), moving from C s configurations through the C 3v geometry. 4 Additionally, Wen and Meyer reported the NO overtone IR spectra of NO−CH 4 and concluded that the molecular complex on average resides in an orientation where NO is perpendicular to the face of CH 4 with C s symmetry.…”

“…This renders NO as a prevalent byproduct of many anthropogenic sources such as motor vehicles, industrial and commercial fuel combustion, and utilities . As such, there has been considerable interest in studying the chemical reactions of NO with other atmospheric species, including rare gas atoms, diatomics, and alkanes. − Methane (CH 4 ) is a potent greenhouse hydrocarbon and is the main component of natural gas. Much of the CH 4 in the atmosphere is also derived from a range of anthropogenic activities, such as extensive livestock farming, fossil fuel extraction, and combustion.…”

“…The study of open-shell complexes is of fundamental interest due to their ability to form interactions that are intermediate between van der Waals forces and chemical bonding. , Open-shell reactive diatomic species such as NO provide important spectroscopic and dynamical signatures that test the accuracy of predictive potential energy surfaces . Previous experimental and theoretical studies have investigated the intermolecular interactions of NO with polyatomic partners, which often exhibit van der Waals interactions and Jahn–Teller distortions, including NO–CH 4 . − …”