The (NO)2 dimer and its ions: Is the solution near?

Abstract

In this manuscript the available experimental and theoretical information on (NO)2 and its ions is summarized and reviewed, and high resolution two photon, one color photoelectron spectra of the dimer are presented. The new spectra yield a wealth of information about the dimer cation, including possibly all six of its vibrational frequencies as well as several anharmonic intermode constants. The most consistent interpretation of the contradictory data available in the literature and of the present results is to assign the gas phase photoelectron spectrum to cis–cis ionization, and the matrix spectra to the trans (NO)2+ ionic ground state. The similarity between the observed matrix frequencies with the gas phase values reflects the insensitivity of the NO stretching fundamentals of the dimer cation to conformation, a point of view supported by the results of theoretical calculations. Despite the problems of theory in computing properties of the (NO)2 species, the experimental structures and molecular constants appear to be in an acceptable agreement with the most recent computational results.

The puzzling infrared spectra of the nitric oxide dimer radical cation: a systematic application of Brueckner methods

“…They also concluded that because of Franck± Condon factors, the cis ONNO ‡ should be observed in the ZEKE spectrum. Urban et al agreed with them and stated that because of poor Franck± Condon factors, the trans cation ground state should not be accessible in their gas phase photoionizaion experiments [9]. Therefore the theoretical study of the cis structure may be as important as the trans.…”

“…Since 1990, the NO dimer and its ionic counterparts have been extensively studied at several experimental laboratories [1± 9], but assignments for the infrared spectrum are still controversial (table 1). For example, the prominent infrared absorption 1619 (or 1618) cm ¡1 has been assigned [2] aş 5 corresponding to the asymmetric N± O stretching vibrational mode of the trans anionic ONNO ¡ , assigned to the trans ONNO ‡ cation [4,7], and assigned to the cis ONNO ‡ [8,9]. The experimental frequencies of other modes are not conclusively identi® ed either (see table 1).…”

“…This 958 Y. Xie and H. F. Schaefer III value is much smaller than those predicted by the DFT methods, which results fall in the range from 1949 to 2058 cm ¡1 . But the Brueckner predictions for the harmonic vibrational frequencies do lie in the region of the experimental spectra features (1589± 1619 cm ¡1 ) [8,9], which have been assumed to be fundamentals. The other frequencies also agree with the recent experimental assignments (see table 4) [9].…”

Direct measurement of the binding energy of the NO dimer

“…The smallest frequency normal mode of ͑NO͒ 2 is 4 , which is 117 cm Ϫ1 for the cis isomer 14 and 79 cm Ϫ1 for the trans isomer. 15 If we were observing dissociation from vibrational hot bands, we would expect to see separate rings for each vibrationally excited state, as the detection system can easily resolve rings corresponding to an energy spacing of Ͼ10 cm Ϫ1 . Since we do not observe such rings, the only remaining source of internal energy for ͑NO͒ 2 is rotational excitation.…”

Multiconfiguration SCF electric field gradients in (NO)2 and its singly charged ions

“…An extensive view of the earlier work on various aspects of the behavior of these dimer species can be found, for example, in the introductions of [1,2], as well as in recent papers dealing separately with the neutral [3], cationic [4] and anionic forms [5]. The NO dimer has also been studied by a number of spectroscopic techniques including those producing microwave transitions [6][7][8].…”