14N Quadrupole Coupling Research Articles

Microwave spectra and structure of an isoxazole–CO Van der Waals complex

The microwave spectra of an isoxazole–CO complex and four of its isotopomers have been measured between 7 and 18 GHz with a pulsed nozzle Fourier transform microwave spectrometer. Rotational constants, centrifugal distortion constants, and N14 quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. The permanent electric dipole moment has been determined from Stark effect splittings. A unique structure has been found for the complex by taking both the moments of inertia of the isotopomers and the quadrupole splittings into account. The complex is planar, with CO lying approximately radially away from nitrogen in the isoxazole ring.

Rotational spectra and molecular structures of N-nitroso-derivatives of pyrrolidine, piperidine, and morpholine and their isoelectronic formyl analogues

Three isoelectronic pairs of compounds, N-formyl- and N-nitrosopyrrolidine, piperidine, and morpholine, have been characterized by microwave spectroscopy. Only one conformational isomer is observed in each compound. Both the inertial constants and the 14N quadrupole coupling constants determined here show that the molecular geometries of each isolectronic pair are extremely similar. Rotational spectra of vibrationally excited species of N-nitrosopyrrolidine and N-nitrosopiperidine are assigned. The range of structural variation among the vibrationally excited states is comparable to the structural variation within the isoelectronic pairs of compounds, suggesting that a biological receptor for one compound is likely to bind its isoelectronic analog as well. Ab initio molecular orbital calculations of N-formylpyrrolidine at the MP2/6–31G∗∗//MP2/31G∗∗ level reproduce the observed rotational econstants extremely well with a model that includes essentially planar bonding about the amino N atom.

14N Coupling Parameters in Oxovanadium(IV)−Amine, −Imine, and −Isothiocyanate Complexes Studied by Electron Spin Echo Envelope Modulation Spectroscopy

Electron spin echo envelope modulation (ESEEM) spectroscopy has been applied to several nitrogen-coordinated oxovanadium(IV) complexes. Results for two amine-nitrogen complexes, VO(edda) and VO(gly)2, two imine-nitrogen complexes, VO(salen) and VO(salophen), and one isothiocyanate complex, VO(NCS)42-, are presented. [H2edda = ethylenediamine-N,N‘-diacetic acid, gly- = glycinate, H2salen = N,N‘-bis(salicylidene)ethylenediamine, and H2salophen = N,N‘-bis(salicylidene)-o-phenylenediamine.] The 14N hyperfine coupling (HFC) and quadrupole coupling (NQC) parameters have been determined from computer simulation of the two-pulse and three-pulse ESEEM data recorded at some selected field positions. Resulting NQC parameters are e2qQ = 3.1 MHz (VO(edda)), 2.7 MHz (VO(gly)2), 2.4 MHz (VO(salen)), 2.6 MHz (VO(salophen), and 1.0 MHz (VO(NCS)42-). The isotropic HFC parameters, |Aiso|, show a good correlation to the nitrogen type as 4.98 MHz (VO(edda)), 5.10 MHz (VO(gly)2), 5.83 MHz (VO(salen)), 5.78 MHz (VO(salophen)), ...

Microwave studies of the three conformers of butyl cyanide

Three conformers of n-butyl cyanide have been characterized by microwave spectroscopy. Low and medium resolution studies on static gas samples in Stark-modulated spectrometers revealed the presence of three conformational species and identified them. High resolution pulsed-jet Fourier transform spectra allowed the assignment of individual transitions of each species yielding rotational and centrifugal distortion constants, 14N quadrupole coupling constants, and qualitative relative energies of the species. The anti-anti and gauche(CN end)- anti forms are more stable than the anti-gauche(methyl end) conformer. The 14N quadrupole coupling constants, which appear so different among the different conformers, are consistent with a reference electric field gradient, that in methyl cyanide, rotated to the inertial axes of each conformer of butyl cyanide.

Pure rotational spectrum, quadrupole coupling constants and structure of the dimer of pyrrole

Rotational transitions of a pyrrole dimer have been identified and measured over the 8–18 GHz range using a pulsed-nozzle Fourier-transform microwave spectrometer. In addition to the parent species, nine N15 and D isotopomers have been analyzed. Apart from the N14 and D nuclear quadrupole splittings, the rotational transitions did not show any additional splittings arising from large amplitude motions. Rotational constants, centrifugal distortion constants and, for two isotopomers, N14 quadrupole coupling constants have been fitted to the measured frequencies of rotational transitions. The observed rotational constants are consistent with essentially a T-shaped structure for the dimer. From the rotational constants of all isotopomers, a partial structure of the dimer has been determined. Three structural parameters have been fitted to the differences in the planar moments of inertia between the isotopically substituted species and the parent species. The planes of the two pyrrole monomers form an angle of 55.4(4)° with the nitrogen side of one ring directed to the π electron system of the other ring establishing a weak hydrogen bond. The centrifugal distortion constants of the dimer of pyrrole have been used to estimate the frequency of the van der Waals stretching mode and the dissociation energy of the complex in a pseudodiatomic approximation.

The molecular and electronic states of 1,2,4,5-tetrazine studied by VUV absorption, near-threshold electron energy-loss spectroscopy and ab initio multi-reference configuration interaction studies

The VUV electronic absorption spectrum of 1,2,4,5-tetrazine has been re-investigated, and together with electron energy-loss spectra has led to identification of a number of new excited states. The valence and Rydberg excited states have been studied by multi-reference multi-root configuration interaction studies using MRDCI techniques. Initial studies with the RPA and TDA methods gave almost identical results for the excitation energies, but there is a substantial energy-lowering in the MRDCI calculations, which improves agreement with experiment substantially; these differences are a result of the double, triple and quadruple excited reference configurations included in the reference set of the latter method. The 1ππ ∗ excitations are calculated rather higher than experiment, except for the lowest-lying (weak) 1B 2u state at 5.0 eV. The calculated order for the next three ππ ∗ states is 1B 1u (weak) followed by 1B 2u (strong) and 1B 1u (strong), the last two bands being responsible for the dominant absorption near 7.5 and 8.5 eV. All of this group of four bands involve excitations from the pair of MOs 1b 2g and 1b 1g into the 1a u ∗ and 4b 3u ∗ VMOs. The sequence of nπ ∗ stakes are in a similar order to the ππ ∗ excitations, with respect to the upper state, and the two lowest singlet states, 1B 3u and 1A u are reasonably well determined. The triplet states follow a similar order to the singlets, and again the dominance of the effect of the two lowest VMOs is demonstrated, but considerable differences between the weighting of leading configurations occurs between singlet and triplet manifolds. The non-diagonal TDA method has been used to reconsider the UV-photoelectron spectrum. The ionisation potentials for tetrazine are reinterpreted with the first three bands being regrouped into 1, 2, 2 ionisations respectively. The ground state properties of tetrazine suggest that the NQR spectrum will show a principal axis 14N quadrupole coupling constant larger than the other known azines. The decline in diamagnetic susceptibility anisotropy in tetrazine, relative to benzene and pyridine, is indicative of lower resonance energy.

The Submillimeter Wave Spectrum of Isotopic Methyl Cyanide

The laboratory submillimeter wave rotational spectrum of the 13CH3CN, CH3C13CN, and CH3C15N isotopomers of methyl cyanide has been observed in natural abundance in the 294 to 607 GHz region. The maximum J and K values are 34 and 14, respectively. Fifteen additional CH3CN transitions up to K = 21 were also measured. The transitions of all four species are fitted to a symmetric top Hamiltonian, and the rotation and distortion constants are determined. The 14N quadrupole and spin rotation coupling constants are also calculated and presented. Suggested values for many other parameters, which could not be directly determined from the isotope spectra, are calculated from the normal species values and isotope relationships. The determined and calculated constants should predict the spectrum of the three isotopomers to well over 1 THz accurately enough for astronomical assignments.

33S nuclear quadrupole coupling in the microwave spectrum of methylthiocyanate

AbstractThe rotational spectrum of 33S‐methylthiocyanate in the vibrational ground state was observed in natural abundance by molecular beam Fourier transform microwave (MB‐FTMW) spectroscopy. We were able to determine the rotational and centrifugal distortion constants, the methyl internal rotation parameters, and the 14N and 33S quadrupole coupling constants. The rotational constants were found to be A = 15598.104(83) MHz, B = 4144.5631 (3) MHz and C = 3338.1246(2) MHz. For the torsional ground state (A‐species) the 33S nuclear quadrupole coupling constants were obtained as χaa = −40.513 (4) MHz, χbb = −7.166 (6) MHz and χcc = 47.679(7) MHz. The corresponding constants for the E‐species lines agree with these values within the given error limits. Additionally the rotational and centrifugal distortion constants, the methyl internal rotation, and the 14N quadrupole coupling constants for the 32S‐ and 34S‐isotopomer could be improved.

Nitrogen quadrupole coupling in the microwave spectrum of trifluoronitromethane CF 3NO 2

Several low J microwave transitions of CF 3NO 2 have been measured under high resolution using a pulsed-nozzle Fourier transform spectrometer. The 14N quadrupole coupling constants in the inertial axis system have been accurately determined as eQq aa = −0.641(2) MHz, eQq bb = 0.327(6) MHz and eQq cc = 0.314(6) MHz. These measurements were made in the light of renewed interest in the internal rotation dynamics of CF 3NO 2. The quadrupole coupling constants provide a probe of the bonding at nitrogen. The measured values are consistent with those known for CH 3NO 2, thus providing further evidence for an effective planar geometry of the CNO 2 grouping in CF 3NO 2.

EPR and ENDOR Studies of 1H and 14N Hyperfine and Quadrupolar Couplings in Crystals of l-O-Serine Phosphate after X-Irradiation at 295 K

Single crystals of l-O-serine phosphate, (HOOC)CH(NH3+)CH2OPO3-H, were X-irradiated at 295 K and studied using EPR, ENDOR, and FSE techniques. At this temperature, three carbon-centered radicals were identified. Radical I, the deamination product (HOOC)ĊHCH2OPO3-H, is shown to have undergone a major molecular reorientation upon formation. Radical II is identified to be the product (HOOC)CH(NH3+)ĊHOPO3-H. This species exhibits a nonplanar site for the lone electron density. This deviation from planarity is ascribed to electrostatic interaction between the lone electron orbital and lone pairs centered on the neighboring oxygen atom. Hyperfine interaction with the β-nitrogen of the amino group is observed. Both the 14N hyperfine and quadrupole coupling tensors are determined and the signs of the hyperfine coupling tensor principal values are deduced from interpretation of the quadrupole tensor employing the Townes−Dailey approach. The β14N isotropic interaction is well described by a Heller and McConnell type cos2 θ rule. The results found in the present work, together with other recent observations, yield estimated values of the constants B0 = 1.0 MHz and B2 = 34.5 MHz. Radical III exhibits the structure (HOOC)CH(NH3+)ĊH2, formed by scission of the phosphate−ester bond at the carbon side. No β14N coupling is observed for this radical due to a dihedral angle close to 90°. Possible mechanistic routes for the formation of these radicals are discussed in comparison with previously published data on serine and other alkyl phosphate derivatives.

Rotational spectrum and ab initio calculations of N-methylformamide

The microwave spectrum of N-methylformamide has been investigated in the 18–40 GHz frequency range. The rotational spectrum of the conformer with the methyl group cis to the aldehydic oxygen has been assigned. Several rotational transitions of the A sublevel of the ground torsional state have been measured for both normal and N—D isotopic species. The V3 barrier to the methyl group internal rotation has been obtained from the effective pseudo-defect of inertia. The dipole moment and the 14N quadrupole coupling constants have been determined. The results of ab initio calculations performed at the HF and MP2 levels are in agreement with the experimental data

Temperature dependence of the 14N quadrupole coupling constant of isocyanomethane

We report NMR relaxation time measurements of the 14N quadrupole coupling parameter χN of isocyanomethane as a function of temperature in the solution-state mixture of 6.4 mol % H3CN ≡ C, 33.5 mol% d6-ethylene glycol, and 60.1 mol % d6-ethanol. We obtain values for the molecular correlation times using the T2/T1 method, which is a function only of the spectral density functions. The results demonstrate a significant, and approximately quadratic, temperature dependence of χN, which shows a minimum value of about 55 kHz at 222 K; the largest value measured was 125 kHz at 161 K. The results are compared with, and discussed within the context of, more traditional methods. The value of χN for neat isocyanomethane in the solid phase was measured to be 26.3 kHz.

Hydrogen bonding to dimethylamine: the microwave spectrum and structure of the dimethylamine-water complex

a- and c-type rotational transitions have been observed for four isotopomers for the dimethylamine-water complex using a Fourier-transform microwave spectrometer. The rotational constants of the four isotopomers are consistent with a structure in which the water acts as a hydrogen bond donor to the dimethylamine ( R NO = 2.82 A ̊ ). The dipole moment is 2.78 (1) D. The 14N and 17O nuclear quadrupole coupling constants were measured and their projections onto the inertial axes are compared with predictions based on structural models.

The microwave spectrum of the thiazole-argon van der Waals complex

The rotational spectra of the 32S and 34S isotopomers of the thiazole-argon complex were measured with molecular beam Fourier transform microwave spectrometers, assigned, and analyzed. The rotational constants are A′ = 3426.1406(14) MHz, B′ = 1221.42799(40) MHz, and C′ = 1136.88313(41) MHz for 32S-thiazole-argon and A′ = 3373.80605(63) MHz, B′ = 1211.00475(64) MHz, and C′ = 1122.03600(25) MHz for the 34S isotopomer. A structure is proposed. The 14N quadrupole coupling constants X aa = 2.39017(41) MHz, X bb = −2.441(16) MHz, X cc = 0.051(16), X ab = −0.75(27) MHz, and X bc = 2.591(33) MHz could be extracted from the spectra of the 32S-thiazole-argon complex. For the 34S isotopomer X aa = 2.3707(25) MHz, X bb = −2.407(37) MHz, X cc = 0.037(37) MHz, and X bc = 2.56(13) MHz were derived.

The Microwave Spectrum of the Isothiazole‐Argon van der Waals Complex

AbstractUsing molecular beam Fourier transform microwave spectroscopy I measured and assigned the rotational spectra of the 32S‐ and 34S‐isothiazole‐argon complexes. The rotational constants are A = 3479.53543(15) MHz, B = 1225.68820(9) MHz, and C = 1154.69246(11) MHz for 32S‐isothiazole‐argon and A = 3428.10063(23) MHz, B = 1216.56872 (19) MHz, and C = 1140.87783(24) MHz for the 34S‐isotopomer. The 14N quadrupole coupling constants χaa = 1.3824(17) MHz, χbb = 0.9126(17) MHz, χcc = ‐2.2951(17) MHz, and χbc = 2.804(35) MHz could be extracted from the analysis of the spectra. For the 34S‐isotopomer I derived χaa = 1.3907(17) MHz, χbb = 0.9242(20), χcc = ‐2.3148(20) MHz, and χbc = 2.829(55) MHz. The spectroscopic constants were used to propose a structure of the isothiazole‐argon van der Waals complex. The argon atom is located 3.55 Å above the ring plane.

The microwave spectrum and structure of the pyridine–CO complex

Rotational spectra of five isotopomers of the pyridine–CO complex with D, 13C, 15N, and 18O have been measured over the 8–18 GHz frequency range. Rotational constants, centrifugal distortion constants, and 14N quadrupole coupling constants have been fitted to the measured transition frequencies of each isotopomer. Two different structures have been found to be compatible with the moments of inertia of all isotopomers. Both have CO lying in the plane of pyridine approaching simultaneously N and ortho-H but with two different orientations of CO.

Rovibrational dependence of the ab initio nuclear quadrupole coupling constants in the X 3Σ − state of NH

The 14N and 2H quadrupole coupling constants of rovibrational levels of 14N 1H and 14N 2H in their ground electronic states have been calculated from molecular wavefunctions which explicity describe nuclear motion. For the lowest vibrational states the 14N quadrupole coupling is predicted to be relatively strong and to decrease rapidly with vibrational excitation. The deuteron coupling in 14N 2H is found to be weak and its behaviour is similar to that of other first-row hydrides. The change with rotational excitation is unimportant. For about six lowest vibrational levels, the quadrupole hyperfine patterns of 14N 2H in its X 3Σ − state are dominated by relatively strong nitrogen coupling.

The microwave spectrum and molecular structure of ethylisocyanate

We have remeasured the spectrum of the isotopically most abundant species of ethylisocyanate (12CH3 12CH2 14N12C16O), and assigned for the first time the spectra of all 13C-, 15N-, and 18O-monosubstituted isotopomers in their respective natural abundances employing pulsed molecular beam microwave Fourier transform spectroscopy. We used these data to determine the heavy atom r s and r o structure where the most interesting structural parameters, the C-N-C and N-C-O angles, were found to be 138·5° and 176°, with the oxygen atom trans to the ethyl group. Additionally we obtained improved values of the 14N quadrupole coupling constants (in MHz) χ aa = 2·5488(6), χ bb = -1·1717(8), and χ cc = -1·3771(8) of the 12CH3 12CH2 14N12C16O species. From the variation of the coupling constants with isotopic substitution, we also determined the angular position of the coupling tensor in the inertial axis system.

ChemInform Abstract: 14N Quadrupole Coupling Constants and 14N and 19F Spin‐Rotation Coupling Constants of Nitrosyl Fluoride, FNO, and Nitryl Fluoride, FNO2.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Determinationof the 14N quadrupole coupling tensor of an ℓ-alanine single crystal by overtone NMR

The overtone NMR approach was examined for determination of the quadrupole coupling tensor of the 14N nucleus in a single crystal and demonstrated for ℓ-alanine.