Nicholas P. C. Westwood

A valence- and inner-shell electronic and photoelectron spectroscopic study of the frontier orbitals of 2,1,3-benzothiadiazole, C6H4SN2, 1,3,2,4-benzodithiadiazine, C6H4S2N2, and 1,3,5,2,4-benzotrithiadiazepine, C6H4S3N2

Abstract HeI photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, C1s and N1s edges, and S1s synchrotron radiation photoabsorption spectroscopy have been used to probe the occupied and unoccupied valence levels of 2,1,3-benzothiadiazole, 1,3,2,4-benzodithiadiazine, and 1,3,5,2,4-benzotrithiadiazepine. The term values obtained from the S1s, S2s and N1s oscillator strength spectra are sensitive to the aromatic/anti-aromatic character in the thiazyl ring, whereas the first ionization potentials for the latter two molecules remain relatively constant. The combination of valence photoelectron and core excitation results, aided by semi-empirical (MNDO) molecular orbital calculations, provides a useful probe of the frontier molecular orbitals of these aromatic/anti-aromatic molecules.

The 'magnetic bottle' spectrometer as a versatile tool for laser photoelectron spectroscopy.

Abstract This paper discusses the use of ‘magnetic bottle’ spectrometers in laser photoelectron spectroscopy. For electron detection both normal time-of-flight methods as well as zero-kinetic energy electron detection with almost laser-limited resolution can be employed. Photofragmentation processes can be monitored via time-of-flight ion detection. A ‘magnetic bottle’ spectrometer can be successfully combined with a molecular beam. To illustrate some of the features, results for the photodissociation of OCS are presented. Highly rotationally excited CO molecules, and S atoms in their excited 1 D 2 state are produced. Rotationally resolved photoelectron spectroscopy of CO is the key to a better understanding of the dynamics of the photoionisation process.

Geometric and electronic structure of dicyanofuroxan by experiment and theory

Dicyanofuroxan (3,4-dicyano-1,2,5-oxadiazole 2-oxide), the precursor to the novel NCCNO species, has been studied in the solid and gas phases to obtain both structural and electronic information. The solid-state structure determined by X-ray diffraction gives an orthorhombic space group Pna21, with a= 10.2578(14), b= 10.8818(12) and c= 10.2259(15)A. There are two independent molecules with similar geometries in the asymmetric unit. The gas-phase molecule is characterized by HeI photoelectron, HeI and HLα,βγ photoionization and IR spectroscopies. The vibrational data is also supported by a Raman study of the solid. The equilibrium geometry of dicyanofuroxan obtained from ab initio calculations at the HF and MP2/6-31G* levels lends support to the crystallographic result of an asymmetric planar five-membered ring with three quite different N–O bonds, including a very short (and strongly polarized) exocyclic N-oxide group. Nevertheless, both HF and MP2 calculations are in poor quantitative agreement with the solid-state structure. Density functional theory (B3-LYP) is, however, much more in accord with the crystallographic result, as indeed, it is with the vibrational data.

Formation of 1-phosphapropyne, CH3CP, by pyrolysis of dichloro-(ethyl)phosphine: a He(I) photoelectron spectroscopic study

The conditions for the isolation of 1-phosphapropyne, CH3CP, recently detected by microwave spectroscopy in the products of the gas-phase pyrolysis of PEtCl2, have been studied by photoelectron spectroscopy. Although CH3CP is not the major product it can be trapped and separated from other reaction products under controlled conditions and subsequently revaporised. The He(I) photoelectron spectrum of CH3CP has been assigned and compared with that of FCP and HCP and the related nitrogen analogues.

The photoelectron spectra of the chloramines NH2Cl, NHCl2, NCl3 and the methyl chloramines CH3NHCl, CH3NCl2, and (CH3)2NCl

The complete series of chlorinated and methylated derivatives of ammonia have been prepared in the pure state and studied by Hei ultraviolet photoelectron spectroscopy. The nonmethyl derivatives were also studied using NeI radiation. Assignments are made on the basis of observed substitution effects, and the spectra are compared with the analogous phosphorus compounds. The first band in all the spectra (arising from ionization of the nitrogen lone pair), shows interesting trends both in position and in bandwidth.

Dimerisation of nitrile oxides: a quantum-chemical study

The [3 + 2] and [3 + 3] cyclodimerisation processes of small nitrile oxides, XCNO (X = F, Cl, Br, CN, CH(3)) are investigated by ab initio coupled cluster theory at the CCSD, CCSD(T) and MR-AQCC levels for the first time. The favoured dimerisation process is a multi-step reaction to furoxans (1,2,5-oxadiazole-2-oxides) involving dinitrosoalkene-like intermediates with diradical character. The rate determining step for all but the F-species is the first, corresponding to the C-C bond formation. The kinetic energy barrier depends on the nature of the substituent X, generally increasing with decreasing electronegativity and increasing pi-donor ability of the substituent: F (DeltaG(298) = 0 kJ mol(-1)) < Cl (72) < Br (90) < CH(3) (104) < CN (114) (MR-AQCC(2,2)//UB3LYP/cc-pVTZ). Following initial C-C bond formation, three possible dinitrosoethylene diradical pathways are explored. Two of them are new, and one of them is a low-energy three-step path with implications for cycloreversion, tautomerism and detection of dinitrosoethylene intermediates. Alternative one-step, concerted [3 + 2] and [3 + 3] cyclodimerisation processes leading to 1,2,4-oxadiazole-4-oxides and 1,4,2,5-dioxadiazines have kinetic energy barriers around 100-240 kJ mol(-1) (CCSD//B3LYP), some 1.6 to 2.5 times higher than those leading to furoxans, supporting the experimental observations of furoxan formation as nitrile oxide loss channels during storage, trapping/re-vaporisation and reactions of nitrile oxides. Potential polymerisation initiation processes for NCCNO, involving the 1,2-dipolar NC substituent are also explored.

Synthesis, Spectroscopy and Structure of the Parent Furoxan (HCNO)2

The parent furoxan (1,2,5-oxadiazole 2-oxide), synthesized from glyoxime and NO(2)(g), has been investigated in the gas phase for the first time by mid-infrared and He I photoelectron spectroscopy, and in the liquid phase by Raman spectroscopy. The ground-state geometry has been obtained from quantum-chemical calculations at the B3LYP, MPn (n = 2-4), CISD, QCISD, CCSD, CCSD(T), RSPTn (n = 2,3), MRCI, and MR-AQCC levels using 6-311++G(2d,2p), cc-pVTZ, aug-cc-pVTZ, cc-pCVTZ, and cc-pVQZ basis sets. Furoxan is predicted to be planar, with a strong exocyclic and a relatively weak endocyclic N-O bond. The furoxan moiety is electron rich, indicated e.g. by a large negative NPA charge (-0.46 e). According to various aromaticity indices, furoxan is nearly as aromatic as furan and furazan. Unlike alkyl- and cyano-substituted furoxans, the parent furoxan, upon thermolysis, does not cleave to the monomer nitrile oxide, yielding only HNCO, HCN, CO(2), CO, NO, and H(2)O decomposition products.

High resolution infrared spectroscopy of cyanogen N‐oxide, NCCNO

We report here on the first high resolution infrared absorption spectra of the semistable nitrile oxide, NCCNO. All of the fundamental modes of vibration (except for the lowest‐frequency bending mode) and several combination bands have been measured with a Fourier transform spectrometer at a resolution of 0.005 cm−1. In this paper, we present analyses of υ4, the C–C stretching mode at 714.753 94(6) cm−1, υ6, the NCC bending mode at 403.925 97(6) cm−1, υ6+υ7 at 490.123 62(6) cm−1, and the tentatively assigned υ5+υ6 combination band at 826.291 86(8) cm−1. A simultaneous least squares fit of these four bands gives ground state rotational constants of B0=0.077 085 54(34) cm−1 and D0=4.570(30)×10−9 cm−1.

The high resolution infrared spectroscopy of cyanogen di‐N‐oxide (ONCCNO)

The high‐resolution infrared absorption spectrum of the oxalodinitrile di‐N‐oxide (ONCCNO) molecule has been recorded in the gas phase with a Fourier transform spectrometer at a resolution of 0.003 cm−1. No previous high‐resolution spectra have been recorded for this semistable palindromic molecule. On the basis of the 2:1 intensity alternation in the rotational lines caused by nitrogen nuclear spin statistics, the ONCCNO molecule appears to be linear. A quasilinear structure, however, cannot be ruled out at this stage of the analysis. The ν4 and ν5 fundamental modes at 2246.040 55(23) cm−1 and 1258.475 30(11) cm−1 have been analyzed to give ground state rotational constants of B0=0.042 202 10(96) cm−1 and D0=8.77(70)×10−10 cm−1. By fixing the CN and NO bond lengths to 1.1923 and 1.1730 A, respectively, the C–C bond length was determined to be 1.3329 A using the B0 value. This short C–C bond length is thus similar to that observed for a carbon–carbon double bond.

Structure and stability of fluoronitrile oxide, FCNO: A quantum-chemical study

The equilibrium geometry, stability, and dimerization of the hitherto unknown fluoronitrile oxide, FCNO, are investigated by various ab initio and density functional methods. The equilibrium geometries are obtained from calculations at the B3LYP, MPn(n = 2–4), QCISD, QCISD(T), CCSD, and CCSD(T) levels with basis sets ranging from 6–31G* up to cc-pVTZ. Calculations were also performed at selected levels using larger, cc-pVQZ (B3LYP and MP2) and cc-pV5Z (B3LYP), basis sets. Potential loss processes for FCNO involving either unimolecular isomerization, or dimerization to a five-membered furoxan ring are investigated at the B3LYP/6-311G(2d) level. FCNO is demonstrated to be a feasible synthetic target in the dilute gas phase or matrix.