Infrared diode laser spectroscopy of the NO3 ν3 band

Abstract

The N–O degenerate stretching band ν3 of the NO3 radical has been studied in the gas phase by infrared tunable diode laser spectroscopy. The NO3 radical was generated by the reaction of NO2 with an excess of O3. Zeeman modulation was employed to observe the paramagnetic absorption lines of 14NO3 and 15NO3 in the wavelength regions 1480–1500 and 1463–1479 cm−1, respectively. Only K′′=3n (n denoting an integer) transitions were observed, and the N′′=even members were missing from the K′′=0 manifold. These observations indicate that the NO3 radical belongs to D3h symmetry in the 2A2′ ground electronic state. The observed spectrum was analyzed using a symmetric-top vibration-rotation Hamiltonian including the spin-rotation interaction. The main parameters thus obtained for 14NO3 are B3=0.455 22(11), C3=0.227 13(6), Cζ3=0.044 79(11), q3=0.001 624(33), t3= 0.000 000 458 0(43), B0=0.457 46(12), C0=B0/2 (fixed), εbb=0.0280(27), and εcc=0.1197(36) for v3=1, εbb=0.0277(28), and εcc=0.1117(34) for v=0, and ν0=1492.3929(9), all in cm−1 with one standard error in parentheses. Although these parameters well reproduced the observed spectrum, the following anomalous features were noted: (1) a large εcc spin-rotation interaction constant was required to explain the spin splittings for both the ν3 and ground states, (2) a higher-order vibration-rotation interaction term having Δk=±4 and Δl=±2 needed to be included, with the corresponding interaction constant t3 larger than that of CHF3, and (3) the centrifugal distortion constants and the first order Coriolis coupling constant which were derived did not agree with those calculated assuming a reasonable force field. These anomalies were ascribed to the interaction with a low-lying excited electronic state and, to some extent, with a combination or overtone state. The N–O bond length was calculated from the B0 rotational constant to be 1.240 Å, in good agreement with an ab initio calculated value.

Conflicting Observations Resolved by a Far IR and UV/Vis Study of the NO3 Radical

“…The prominent infrared absorption at 1492 cm À1 was thoroughly investigated by highresolution gas phase spectroscopy almost 25 years ago. [13,19] Although this band was assigned to the degenerate stretching fundamental, n 3 , [12,13,16] recent calculations [11] have challenged that assignment. The absorptions above 1100 cm À1 seem to be almost entirely binary or higher combinations of the three fundamentals n 1 (1053 cm À1 ), [9] n 3 (between 900-1100 cm…”

“…[4,5] Numerous spectroscopic studies have sought to obtain the equilibrium symmetry and the assignment of the vibrational transitions in the ground electronic state. Important recent studies include: 1) photoelectron (PE) spectroscopic studies of both the nitrate anion [6,7] and the NO 3 radical, [8] 2) laser-induced fluorescence (LIF) spectra of the nitrate radical, [9,10] as well as a recent reanalysis of the vibronic levels obtained from these spectra, aided by non-adiabatic computations, [11] 3) the analysis of high-resolution diode laser and FTIR gas phase spectra which were obtained in the region 700 to 1520 cm À1 , [12,13] and later extended to 1300-2800 cm À1 [14][15][16] and to the near infrared, [17] and 4) a neonmatrix FTIR study on NO 3 in the range from 450 to 6000 cm À1 . [18] From the analysis of the recent spectra, D 3h symmetry was determined for the free NO 3 radical in the X $ 2 A 2 ' ground state.…”

Communication: MULTIMODE calculations of low-lying vibrational states of NO3 using an adiabatic potential energy surface

“…It has been the focus of a large body of spectroscopic work due to its complexity and difficulty in making state assignments. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Theoretically, numerous papers have appeared dealing with various aspects of the electronic structure and vibrational analysis [15][16][17][18][19][20][21] and we refer the reader to those papers for details. We note that theoretical work on vibrational assignments done by Stanton has used a model vibronic Hamiltonian, developed by Köppel−Domcke−Cederbaum, 22 with high-level ab initio input and some empirical scaling of the diabatic coupling to obtain good agreement with experiment.…”

“…18 A particularly controversial assignment is the band at around 1492 cm −1 where Stanton's calculations (both adiabatic and vibronic) disagree with the experimental assignment of Hirota and co-workers, who assigned this as the N-O degenerate stretching ν 3 mode. 1 In 2007, Stanton 16 predicted the v 3 fundamental to around 1000 cm −1 and recent calculations 19 put it at 1069 cm −1 , with an estimated uncertainty of ±30 cm −1 ; thus at an energy roughly 400 cm −1 below the 1492 cm −1 band. It should be noted that the QFF has a harmonic frequency of this mode of 1133 cm −1 , which is close to an earlier result from UCCSD(T)/aug-cc-pVDZ a) Authors to whom correspondence should be addressed.…”

On Walking in the Footprints of Giants

“…Eizi Hirota and his colleagues (79)(80)(81) thoroughly characterized the gas-phase spectrum of ground-state NO 3 and definitively ascertained it to possess D 3h symmetry. Because of strong interaction of the ground state with two low-lying excited electronic states, the spectrum includes several prominent combination bands that are still incompletely assigned.…”