Inertial Axis System Research Articles

Spectroscopic studies of the $\tilde{A}$Ö$\tilde{X}$X̃ electronic spectrum of the β-hydroxyethylperoxy radical: Structure and dynamics

The jet-cooled Ã-X̃ near IR origin band spectra of the G(1)G(2)G(3) conformer of four β-hydroxyethylperoxy isotopologues, β-HEP (HOCH(2)CH(2)OO), β-DHEP (DOCH(2)CH(2)OO), β-HEP-d(4) (HOCD(2)CD(2)OO), and β-DHEP-d(4) (DOCD(2)CD(2)OO), have been recorded by a cavity ringdown spectrometer with a laser source linewidth of ~70 MHz. The spectra of all four isotopologues have been analyzed and successfully simulated with an evolutionary algorithm, confirming the cyclic structure of the molecule responsible for the observed origin band. The analysis also provides experimental à and X̃ state rotational constants and the orientation of the transition dipole moment in the inertial axis system; these quantities are compared to results from electronic structure calculations. The observed, broad linewidth (Δν > 2 GHz) is attributed to a shortened lifetime of the à state associated with dynamics along the reaction path for hydrogen transfer from the OH to OO group.

A code for analysis of the fine structure in near-rigid weakly-bonded open-shell complexes that consist of a diatomic radical in a [formula omitted] state and a closed-shell molecule

A code for analysis of the fine structure in near-rigid weakly-bonded open-shell complexes that consist of a diatomic radical in a [formula omitted] state and a closed-shell molecule

A contribution to the structure of the van-der-Waals complex phenylacetylene–argon by microwave spectroscopy and quantum chemistry

Continuing earlier work on phenylacetylene (PhAc), we have measured the molecular rotational spectra of 24 isotopologues of the van-der-Waals complex phenylacetylene–argon (PhAc–Ar) by molecular beam Fourier transform microwave spectroscopy. All atomic positions could be singly substituted or in combination with others. The inertial moments were employed to calculate the molecular structure by recent methods ( r m, r s). In contrast to PhAc, the complex PhAc–Ar is an unfavorable object for this type of structure determination: (1) several atoms near the center of mass have very small coordinates in the principal inertial axis system (PAS), (2) obviously neither the simple treatment of the rovib contributions to the inertial moments by the r s-method (by the dominance of inertial moment differences) nor the more sophisticated one by the r m-method(s) can take proper account of the influence of the large amplitude vibrational motion of the weakly bound argon. Detailed quantum-chemical calculations (optimizing the full complex) succeeded in getting a better molecular structure although different methods yielded an argon position with a somewhat larger than usual range of uncertainty. Most interestingly, the results hinted at a very slight deformation of the PhAc moiety. However, with the methods presently available, a possible non-planarity of the PhAc ligand can neither be proved nor disproved. Borrowing a few elements from the quantum-chemically obtained structure and applying them as constraints within the r m ( 1 ) method eventually lead to a tolerable molecular structure also by the method of fitting inertial moments.

Rotational spectroscopy and dipole moment of cis-cis HOONO and DOONO

The rotational spectrum of cis-cis HOONO has been studied over a broad range of frequencies, 13-840 GHz, using pulsed beam Fourier-transform microwave spectroscopy and room-temperature flow cell submillimeter spectroscopy. The rotational spectrum of the deuterated isotopomer, cis-cis DOONO, has been studied over a subset of this range, 84-640 GHz. Improved spectroscopic constants have been determined for HOONO, and the DOONO spectrum is analyzed for the first time. Weak-field Stark effect measurements in the region of 84-110 GHz have been employed to determine the molecular dipole moments of cis-cis HOONO [mu(a) = 0.542(8) D, mu(b) = 0.918(15) D, mu = 1.07(2) D] and DOONO [mu(a) = 0.517(9) D, mu(b) = 0.930(15) D, mu = 1.06(2) D]. The quadrupole coupling tensor in the principal inertial axis system for the 14N nucleus has been determined to be chi(aa) = 1.4907(25) MHz, chi(bb) = -4.5990(59) MHz, chi(ab) = 3.17(147) MHz, and chi(cc) = 3.1082(59) MHz. Coordinates of the H atom in the center-of-mass frame have been determined with use of the Kraitchman equations, /aH/ = 0.516 A and /bH/ = 1.171 A. The inertial defects of HOONO and DOONO are consistent with a planar equilibrium structure with significant out-of-plane H atom torsional motion. Comparisons of the present results are made to ab initio calculations.

Microwave Measurements of 14N and D Quadrupole Coupling for (Z)-2-Hydroxypyridine and 2-Pyridone Tautomers

Rotational spectra for the two tautomers (Z)-2-hydroxypyridine and 2-pyridone and their deuterated isotopomers were measured in the microwave range between 4 and 14 GHz using a pulsed beam Fourier transform microwave spectrometer. Nitrogen and deuterium quadrupole hyperfine structure was completely resolved for many of the observed transitions, and the measured 14N quadrupole coupling tensors are quite different for these two tautomers. The eQqcc(N) values have opposite signs. The 14N quadrupole coupling strengths for (Z)-2-hydroxypyridine in the principal inertial axis system are as follows: eQqaa(N) = −0.076(11), eQqbb(N) = −2.283(6), and eQqcc(N) = 2.359(6) MHz. The 14N and D nuclear quadrupole coupling strengths for (Z)-2-deuteriohydroxypyridine in the principal inertial axis are eQqaa(N) = −0.1465(4), eQqbb(N) = −2.2045(4), and eQqcc(N) = 2.3510(4) MHz and eQqaa(D) = −0.0250(9), eQqbb(D) = 0.1699(4), and eQqcc(D) = −0.1449(4) MHz. The 14N quadrupole coupling strengths for 2-pyridone in the principal inertial axis system are eQqaa(N) = 1.496(4), eQqbb(N) = 1.269(4), and eQqcc(N) = −2.765(4) MHz. The 14N and D nuclear quadrupole coupling strengths for 1-deuterio-2-pyridone in the principal inertial axis system are as follows: eQqaa(N) = 1.511(2), eQqbb(N) = 1.249(5), and eQqcc(N) = −2.759(5) MHz and eQqaa(D) = −0.110(7), eQqbb(D) = 0.354(6), and eQqcc(D) = −0.244(6) MHz. New, improved, experimental rotational constants were obtained for the H and D isotopomers of both tautomers. Kraitchman analysis indicates the “tautomeric” hydrogen atom is at a distance of 2.653(2) Å in 2-hydroxypyridine and a distance of 2.124(1) Å in 2-pyridone from the centers of mass of the two tautomers, respectively. The DFT calculated eQq(N) values for both the tautomers and the deuterated tautomers are in good agreement with the present experimental values. The Townes−Dailey model has been used to analyze the new quadrupole coupling data of the tautomers and the results are presented in terms of nitrogen atom p-orbital occupation numbers.

An investigation of the quadrupole coupling of two N nuclei in the complex pyrimidine–Ar by molecular beam Fourier transform microwave spectroscopy

The quadrupole coupling hyperfine structure of two equivalent 14N nuclei in the rotational spectrum of pyrimidine–Ar complex has been investigated in the range of 6–18 GHz using molecular beam Fourier transform microwave spectroscopy. From the analysis of the hyperfine structure the elements χ aa , χ bb , χ cc and χ bc of the quadrupole coupling tensors in the principal inertial axis system of the complex have been determined. A comparison of the quadrupole constants of the complex with those of bare pyrimidine indicates that these constants are consistent with the structure of the complex previously reported, with the Ar atom tilted ca. 5° towards the two N atoms. This configuration is supported by ab initio calculations.

Evidence concerning the relative nucleophilicities of non-bonding and π-bonding electrons in furan from the rotational spectrum of furan···ClF

The rotational spectra of the three isotopomers C4H4O···35ClF, C4H4O···37ClF and C4D4O···35ClF of a complex formed between furan and chlorine monofluoride have been observed by means of a pulsed-nozzle, Fourier-transform microwave spectrometer fitted with a fast-mixing nozzle to preclude any chemical reaction among the component gases. Transitions allowed by all three components µa, µb and µc of the electric dipole moment were observed, those associated with µc exhibiting a small doubling (ca. 20 kHz). Spectral analysis yielded rotational constants, centrifugal distortion constants and (for the isotopomers C4H4O···35ClF and C4H4O···37ClF) all five independent components of the Cl nuclear quadrupole coupling tensor χαβ (α, β = a, b, c). Diagonalisation of the tensor χαβ led to the values of the components χii (i = x, y, z) in the ClF principal axis system and the direction cosines θαz (α = a, b, c) relating the ClF axis (z) to the principal inertial axis system. The geometry obtained by fitting the principal moments of inertia of the three observed isotopomers is consistent with the θαz values and has the ClF subunit approximately perpendicular to one of the C(2)–C(3) bonds of furan. The end δ+Cl of ClF thus appears to interact with the π electron density between these two atoms. The distance of the Cl atom from the point of intersection of the extrapolated ClF axis with the ring is r(···Cl) = 2.726 A. The doubling of the c-type transitions is interpreted in terms of a motion in which the interaction switches from C(2)–C(3) to C(2′)–C(3′). The contrast between the angular geometries of furan···HCl and furan···ClF is discussed in the context of the relative nucleophilicities of the n-pair and the aromatic π-electrons of furan.

Microwave Spectrum, Quadrupole Coupling Constants, and Dipole Moments of 1-Chloro-2-fluorobenzene

The microwave spectrum of 1-chloro-2-fluorobenzene has been reinvestigated in the frequency region 7–14 GHz by a molecular beam Fourier transform microwave spectrometer. A least-squares analysis of the observeda- andb-type transition frequencies gave rotational and quartic centrifugal distortion constants and the components of the nuclear quadrupole coupling tensor in the principal inertial axis system[formula]for 1-[35Cl]chloro-2-fluorobenzene. The dipole moments were determined to be μa= 2.28(85)D and μb= 1.47(30)D.

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.

Calculating nuclear quadrupole coupling constants for van der Waals complexes

The nuclear quadrupole coupling constants of van der Waals complexes contain valuable information on intermolecular forces. In the past, the coupling constants have been interpreted in terms of angular expectation values, involving the projections of the monomer coupling constant onto the inertial axes of the complex. However, this is a significant approximation. The present paper develops the theory needed to calculate nuclear quadrupole splittings without defining an inertial axis system for the complex. The theory is applied to Ar-CO2, Ar-HCl and Ar-N2 as test cases. An improved value of the quadrupole coupling constant of the N2 monomer is obtained, based on the microwave spectra of Ar-N2 and Kr-N2.

On the choice of inertial axes for interpreting spectroscopic properties of van der Waals complexes

Properties such as the dipole moments and nuclear quadrupole coupling constants of van der Waals complexes are important in the determination of intermolecular potential energy surfaces from high-resolution spectra. The properties are often interpreted in terms of angular expectation values. It is shown that, when calculating such properties, it is important to use an inertial axis system that satisfies the Eckart conditions. Projections onto other axes, such as the intermolecular vector or the instantaneous principal axes, can lead to substantial errors when the individual monomers have large moments of inertia.

Can the nonlinearity of hydrogen bonds B…HX be measured precisely? A method from rotational spectroscopy and its application to 2,5-dihydrofuran…HCl

Rotational constants, centrifugal distortion constants and the complete Cl-nuclear quadrupole coupling tensor referring to the principal inertial axis system have been determined for the ground state of 2,5-dihydrofuran…HCl from its rotational spectrum. The complex has C s symmetry, with the hydrogen bond completing a pyramidal arrangement at the oxygen atom of 2,5-dihydrofuran. The coupling tensor allows a good approximation to the equilibrium angle α az between the HCl( z) axis and the a axis, and thence the nonlinearity θ=9.5° of the hydrogen bond, to be determined. The results are rationalised in terms of a refinement of a simple model of the hydrogen bonds.

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.

Microwave spectroscopic investigation of thionyl chloride, SOCl2: hyperfine constants and harmonic force field

The rotational spectrum of the most abundant isotopomer of thionyl chloride, 32S16O35Cl2, has been recorded in the frequency range 7.8–24.7 GHz using a pulsed molecular-beam microwave spectrometer. 262 lines of 27 rotational transitions (b- and c-type) have been used to determine rotational and quartic centrifugal distortion constants and the complete 35Cl nuclear quadrupole coupling tensor, as well as spin–rotation coupling constants. Whereas the diagonal quadrupole coupling constants in the inertial axis system agree with those from previous microwave studies, disagreement is observed for the principal values χzz(–63.8 MHz) and η(–0.0143). However, the value of χzz is in good agreement with those derived from solid-state NQR measurements. The centrifugal distortion constants together with vibrational wavenumbers and amplitudes have been used to refine the harmonic force field. The principal quadrupole coupling constants and the force constants are compared with those of related molecules, and the latter have been used to propose a reassignment of two vibrational bands of the related molecule, SO2Cl2.

Non-linear hydrogen bonds and rotational spectroscopy: measurement and rationalisation of the deviation from linearity

When the rotational spectra of hydrogen-bonded dimers B⋯HX (X = Cl or Br) of suitable symmetry are investigated under high resolution by the pulsed-nozzle, Fourier-transform technique, it is possible to determine the complete halogen nuclear quadrupole coupling tensor referred to the principal inertial axis system of the molecule. This tensor can then be diagonalised to give the components χxx, χyy and χzz in the HX axis system and the angle αaz between the direction of the principal inertial axis a and the direction of the HX axis z. A consideration of the values of χxx and χyy establishes that the HX angular oscillation is close to isotropic with respect to the x and y directions. It can then be shown that the angle αaz obtained is unaffected by the zero-point angular oscillation of the HX subunit. This result allows the deviation θ of the hydrogen bond B⋯H—X from linearity to be determined.Values of θ determined for the series of hydrogen-bonded dimers B⋯HX, where B is furan, sulfur dioxide, 2,5-dihydrofuran, oxirane, thiirane and formaldehyde and X is either 35Cl or 79Br, are considered. θ is found to vary significantly along this series and this variation is discussed in terms of a simple model of the complex involving a secondary interaction between the X atom and the subunit B.

Microwave spectrum of acetaldehyde-1- d1: Deuterium quadrupole splittings and internal rotation analysis

The rotational spectrum of acetaldehyde-1- d 1 (CH 3CDO) has been investigated between 8 and 40 GHz with a conventional Stark-modulated spectrometer. All observed transitions were split by the internal rotation of the methyl group. Rotational constants, centrifugal distortion constants and internal rotation parameters have been fitted to the measured transition frequencies. The deuterium quadrupole splittings of seven low- J transitions have been studied with a pulsed-nozzle beam microwave Fourier transform spectrometer. The deuterium quadrupole coupling constants have been determined in the principal inertial axis system.

The microwave spectrum of the FO2radical

Over 250 mm wave transitions of FO2 in its ground 2 A″ vibronic state have been detected using the F + O2 + M reaction as a source of the radical. The lines have been assigned to a-type transitions involving levels with N between 7 and 15 and show resolved or partially resolved fine structure and hyperfine structure. A combined fit with 445 infrared combination differences yielded rotation, distortion, spin-rotation and hyperfine parameters. The ground state spin-rotation tensor components that were determined were used to derive more accurate spin-rotation parameters for several vibrationally excited levels, based on spin splittings from previously published FTIR and diode laser results. The hyperfine parameters are in quite good agreement with, and much more accurate than, those determined from earlier E.S.R. results on FO2, when the latter are rotated into the principal inertial axis system of the molecule.

Nitrogen-14 quadrupole coupling constants in N 2O 3 by isotopic labelling

Several microwave transitions of O 15N…NO 2 and ON… 15NO 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 determined for both nitrogen atoms. The quadrupole coupling constants for ON … 15NO 2 are eQq aa = −0.5203(20) and eQq bb = −4.1981(19) MHz, and for O 15N…NO 2 are eQq aa = −1.7999(13) and eQq bb = 0.0808(17) MHz. The 14N quadrpole coupling constants determined by Kukolich for the main species ON…NO 2 should be reversed with respect to the NO and NO 2 groups. Combining the present data with the main species constants allows the complete quadrupole coupling tensor to be estimated for the NO 2 group; the tensor in N 2O 3 lies within 2° of the N…N bond direction and within 0.4° of the bisector of the ONO angle. Spin-rotation effects are significant for nitrogen in the NO group; C aa is determined to be 8 ± 2 kHz for 14N in ON… 15NO 2 and −15 ± 3 kHz for 15N obtained directly from splittings in the spectrum of O 15N… 15NO 2.

Nitrogen-14 Quadrupole Coupling Constants in Propanedinitrile, CH2(CN)2, by Microwave Spectroscopy

Nitrogen-14 quadrupole hyperfine structure has been resolved in the microwave spectrum of propanedinitrile, CH2(CN)2. Measurements between 5 and 30 GHz by Fourier transform and Stark modulation spectroscopy have been analysed. The general matrix elements for nuclear quadrupole interaction in molecules containing two equivalent coupling nuclei have been derived as simple formulae. The quadrupole coupling constants determined for the inertial axis system of CH2(CN)2 are χaa = - 2.368(28) MHz, χbb = 0.318(20) MHz, and χcc = 2.050(20) MHz. These values are used to establish that the electric field gradient tensor at nitrogen is cylindrically symmetric about the C≡N bond with a value of χCN = - 4.09(4) MHz, close to values found for other organic cyanides.

The EPR spectrum of the HO 2 radical and determination of ground state parameters

The detection of lines in the gas phase EPR spectrum of the HO 2 radical at a frequency of 9 GHz is reported. The measurements are combined with the data from the far infrared LMR spectrum and with the zero-field frequencies in the microwave spectrum and fitted to determine a very nearly complete set of ground state parameters. A theoretical relationship is used in order to determine all the non-zero components of the spin rotation tensor ϵ in the principal inertial axis system. The spin-rotation interaction is interpreted in terms of the admixture of both the A ̃ 2A′ state and higher 2 A′ states. The three principal components of the electron spin g-tensor are determined by the magnetic resonance data and are found to be well predicted by Curl's relationship from the spin-rotation tensor. In addition, it was found to be necessary to include terms from the rotational Zeeman effect to obtain a satisfactory fit of the data. Two of the principal components of the rotational g-tensor are determined; the valuefor g r aa is shown to be consistent with Coriolis mixing of the A ̃ 2A′ state. The hyperfine parameters determined in the fit of the microwave data are interpreted in terms of a simple molecular orbital description. The results suggest that about 27% of the spin density is on the central O atom.