Nuclear hyperfine coupling interactions in the rotational spectra of the linear and bent isomers of HF-N2O

The rotational spectra of six isotopomers of the linear and bent isomers of HF-N₂O have been collected in the 7-18 GHz region with a Fourier transform microwave spectrometer. The nuclear hyperfine structure in the spectra produced by HF spin-spin coupling interaction and nuclear quadrupole coupling interactions due to the D nucleus of DF and the nuclei of N₂O have been resolved and analyzed. In the linear isomer, H in HF is bonded to the terminal N in N₂O. The NF bond lengths are 2.9808(2) Å for the HF-containing isotopomers and 2.9732(2) Å for the DF-containing isotopomers. The zero point angles are 23.1° for HF and 31-34° for N₂O. The hyperfine constants suggest that the HF bond is lengthened by 0.0105 Å upon complexation and that the electric field gradients of the two nitrogen nuclei in N₂O are perturbed differently in the complex. In the bent isomer, the hydrogen bond is formed between HF and O in N₂O. The intermolecular distances are 3.4942(2) Å for the HF-containing isotopomers and 3.4436(2) Å for the DF-containing isotopomers, with HF and N₂O forming angles of 34° and 46°, respectively, with the intermolecular axis. The nuclear quadrupole coupling constants of the two nitrogen nuclei do not indicate electric field gradient perturbation in this isomer.     

Microwave spectra of eight isotopic modifications of 1-chloro-1-fluoroethylene

The rotational spectra of eight isotopomers of 1-chloro-1-fluoroethylene in the 6-22 GHz region have been collected and analyzed. Each rotational transition is split into hyperfine components by the chlorine (either ³⁵Cl or ³⁷Cl) nuclear quadrupole coupling interaction and additionally, one or more smaller interactions such as the spin-rotation interaction due to the fluorine atom, hydrogen-hydrogen spin-spin coupling interactions, and in appropriately substituted species, the deuterium nuclear quadrupole hyperfine interaction. The rotational constants derived from these isotopomers allow the determination of average and Kraitchman substitution structures for 1-chloro-1-fluoroethylene, whereas the availability of the diagonal chlorine nuclear quadrupole coupling constants for all the isotopomers provides complete quadrupole coupling tensors for both ³⁵Cl and ³⁷Cl. In the course of this work, the rotational spectrum of an excited vibrational state of the normal isotopomer was observed, which ab initio calculations suggest should be assigned to ν₉=1, an in-plane bending motion at the CFCl end of the molecule.     

Rotational spectra and internal dynamics of Ne—H2S

Rotational spectra of 15 isotopomers of the Ne-H₂S van der Waals complex were measured in the frequency range 4–22 GHz using a pulsed molecular beam Fourier transform spectrometer. Two K = 0 progressions were observed for each of the symmetric isotopomers (with H₂S or D₂S). This doubling is attributed to an internal rotation motion of the H₂S subunit within the complex. These two states can be correlated with the 0₀₀ and 1₀₁ rotational states of free H₂S and D₂S. By contrast, symmetry constraints no longer apply to isotopomers with DHS. The excited internal rotor state is no longer metastable, and only one K = 0 progression could be observed. The rotational constants obtained were compared with those of Ar-H₂S and Ar—H₂O. The ground state rotational constant remained almost constant upon substitution of H with D, showing an unusual isotope effect, similarly to a previous observation in Ar-H₂S (GUTOWSKY, H. S., EMILSSON, T., and ARUNAN, E., 1997, J. chem. Phys., 106, 5309). This behaviour is in agreement with the ab initio study by OLIVEIRA, G. D., and DYKSTRA, C. E., 1999, J. chem. Phys., 110, 289. An approximate substitution analysis was carried out to deduce structural information from the ground state rotational constants. Nuclear quadrupole hyperfine structures were observed and resolved or partially resolved for isotopomers containing ³³S and D, respectively, and the corresponding nuclear quadrupole coupling constants were determined. These were used to derive information about the internal dynamics of the dimer. Different sensitivities of the quadrupole coupling constants of D and ³³S to the extent of out-of-plane motion were revealed.     

The Molecular Structure of Morpholine and the Morpholine–H2O Complex Determined by FT Microwave Spectroscopy

The rotational spectrum of the morpholine–H₂O complex was measured and assigned using a Balle–Flygare type FT microwave spectrometer. Rotational, quartic centrifugal distortion, and¹⁴N quadrupole coupling constants were determined, and a N … H–O hydrogen-bonded structure was found to be consistent with the derived molecular parameters. Additionally, the rotational spectrum of the¹³C and¹⁵N isotopomers of the morpholine monomer were measured in natural abundance to determine itsr₀structure and a partial heavy atomr_sstructure.     

Effective orientation of water in 1,4-dioxane···water: the rotational spectrum of the H2 17O isotopologue

The measurements of the rotational spectrum of 1,4-dioxane-water complex, performed by pulsed jet Fourier transform microwave spectroscopy, have been extended to the 7–18.5 GHz frequency region and to two additional isotopologues, 1,4-dioxane···H₂ ¹⁷O and 1,4-dioxane···HOD. The effective orientation of water in the complex has been obtained from the ¹⁷O quadrupole coupling constants.     

Rotational spectra of the Kr-H2O van der Waals complex

Rotational spectra of the Kr-H₂O van der Waals complex were measured in the frequency range 4–19 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. The isotopomers studied include those of H₂O, HDO, D₂O, H₂ ¹⁷O and H₂ ¹⁸O with the six most abundant isotopes of Kr. A tunnelling splitting due to a large amplitude internal motion of the H₂O subunit that exchanges bonded and non-bonded hydrogen atoms was observed. Nuclear quadrupole hyperfine structure was resolved and measured for the complexes containing ⁸³Kr, D, and ^l7O, and the corresponding nuclear quadrupole coupling constants were determined. These were used to estimate structural parameters and to derive information about the intermolecular dynamics. The results, and in some instances the spectroscopic constants themselves, were compared with experimental and theoretical data previously reported for Ar-H₂O.     

The microwave spectrum of cyanophosphine, H2PCN

The rotational spectra of cyanophosphine, H₂PCN, have been measured between 10 and 42.5 GHz by Fourier transform microwave spectroscopy. The rotational constants, centrifugal distortion constants, the ¹⁴N quadrupole coupling constant, and the nuclear spin-rotation coupling constants of ³¹P have been determined. Density functional ab initio calculations were performed, and the calculated values of the molecular constants are in excellent agreement with our experimentally determined results. The spectra of three isotopomers were measured, H₂P¹²C¹⁴N, H₂P¹³C¹⁴N, and H₂P¹²C¹⁵N. The derived r₀ structure is quite comparable to the ab initio predicted H₂PCN equilibrium geometry.     

Microwave spectra of the Cl and Cl isotopomers of dichloromethylene: Nuclear quadrupole-, spin-rotation-, and nuclear shielding constants from the hyperfine structures of rotational lines

The rotational spectra of the isotopomers C³⁵Cl³⁷Cl and C³⁷Cl₂ of dichloromethylene in the ground vibronic state were recorded in the range 10-33 GHz using a molecular beam Fourier transform microwave spectrometer. CCl₂ was generated by flash pyrolysis using different precursors. The observed spectra were analyzed to yield rotational and centrifugal distortion constants, as well as the complete Cl nuclear quadrupole coupling tensors and the spin-rotation interaction constants from the hyperfine structure of the rotational lines. With inclusion of data from previous work on the most abundant species C³⁵Cl₂ [N. Hansen, H. Mäder, F. Temps, Phys. Chem. Chem. Phys. (3) (2001) 50-55.] a refined r₀ structure was determined. The spin-rotation interaction constants of all three isotopomers were used to derive ³⁵Cl and ³⁷Cl principal inertial axis nuclear magnetic shielding components which have not yet been determined by NMR spectroscopy.     

Microwave spectrum of NF2

About 350 lines in the microwave spectrum of NF₂ have been measured in various ranges of frequency between 13.0 and 65.2 GHz by using two types of Zeeman effect spectrometers. Complete assignment of all lines has been achieved and, via the general microwave computer program SPINRO, the rotational constants, centrifugal distortion constants, dipole moment, electronic spin-rotation coupling constants, the constants for the coupling of the several nuclear spins with the electron spin and the nitrogen quadrupole coupling constants have all been obtained.By drawing upon the observed vibrational frequencies the average geometry of NF₂ has been evaluated. Force constants and Coriolis coupling constants have also been derived.The values of the spin coupling constants for N and for F indicate that NF₂ is a π-radical with the spin density mainly located on nitrogen. The multiplet patterns indicate that the ground electronic state wavefunction is antisymmetric to rotation about the molecular symmetry axis and so, for a π-radical, identifies the ground state as ²B₁ as has previously been assumed for this molecule.     

The microwave spectrum of cyanoformamide

The spectrum of cyanoformamide, NCCONH₂, has been measured between 18 and 40 GHz using a Hewlett-Packard spectrometer with Stark modulation. The molecule is somewhat unstable and could most conveniently be measured in a flow system. The quadrupole hyperfine structure due to the two nitrogen nuclei has been analyzed for the ground state, and quadrupole coupling constants, rotational constants, and centrifugal distortion constants have been determined for the ground state. A rough determination of the components of the electric dipole moment was possible from the Stark shifts of suitable transitions.     

The pure rotational spectra of the lanthanum monohalides, LaF, LaCl, LaBr, LaI

Pure rotational spectra have been measured for all the major isotopomers of the lanthanum monohalides, LaF, LaCl, LaBr, and LaI, in their ground and (except for ) excited vibrational states. The spectra were observed with a cavity pulsed jet Fourier transform microwave spectrometer in the frequency range 5-24 GHz. The molecules were prepared by laser ablation of La metal and allowing the resulting plasma to react with SF₆, Cl₂, Br₂, or CH₃I precursor in an Ar carrier gas of the pulsed jet. For LaBr this is the first reported spectrum of any kind. Rotational constants, centrifugal distortion constants, nuclear quadrupole coupling constants, and nuclear spin-rotation constants have been determined for all the molecules. Accurate equilibrium (r_e) internuclear distances have given an indication of where the Born-Oppenheimer approximation is beginning to fail. From the centrifugal distortion constants and vibration-rotation (α_e) constants good estimates of the harmonic vibration frequencies and bond dissociation energies have been obtained. The halogen nuclear quadrupole coupling constants indicate the molecules to be highly ionic.     

A molecular beam Fourier transform microwave study of 2-methylpyridine and its complex with argon: structure,methyl internal rotation and 14N nuclear quadrupole coupling

The rotational spectrum of 2-methylpyridine (α-picoline, CH₃C₅H₄N) in the two lowest levels of methyl internal rotation (m=0, ±1) has been recorded in the frequency range from 2 to 15 GHz using a molecular beam Fourier transform microwave spectrometer. The high resolution and sensitivity of this spectroscopic technique allowed resolution of hyperfine structures due to ^l4N nuclear quadrupole coupling with high accuracy and detection of the spectra of the ¹⁵N- and all ¹³C-isotopomers. These investigations considerably extend the results from an earlier study on the normal species (Dreizler, H., Rudolph, H. D., and Mader, H., 1970, Z. Naturforsch., 25a, 25); improved rotational and centrifugal distortion constants as well as all components of the ¹⁴N quadrupole coupling tensor have been obtained. Analysis of the spectra of the isotopomers yielded the ^I4N quadrupole coupling constants χ _cc and χ _aa – χ _bb (for the ¹³C species), the potential parameter V ₃ for the barrier hindering the internal rotation of the methyl group, and, in particular, r_o, r_s r _m ⁽¹⁾ and r _m ⁽²⁾ structural parameters for the molecule. In addition to the microwave studies on the monomer, we have also investigated the rotational spectrum of the weakly bound dimer of normal 2-methylpyridine with Ar. The results obtained for the quadrupole coupling constants and the hindering potential for the internal methyl rotation show that the corresponding parameters are not significantly, or only slightly, changed in the complex.     

Intramolecular hydrogen bonding in chiral alcohols: The microwave spectrum of the chloropropanols

The microwave Fourier transform spectrum of a mixture of 1-chloropropan-2-ol and 2-chloropropan-1-ol has been recorded in the range 5-18 GHz. The spectrum is extremely dense, as a result of both the large number of species present in the supersonic expansion, and the splitting of rotational transitions due to quadrupole coupling. Two conformational isomers of each molecule have been identified, and both the ³⁵Cl and ³⁷Cl isotopomers have been observed for both conformations of 1-chloropropan-2-ol and the most abundant conformation of 2-chloropropan-1-ol. All four observed conformations have weak intramolecular hydrogen bonds between the hydroxyl hydrogen and the chlorine atom. The basis of our assignment is a series of ab initio calculations, performed using the GAUSSIAN package, good agreement being observed between theoretical and experimental values of the rotational constants. Analysis of isotopic substitution and quadrupole coupling effects has reinforced this assignment. First-order centrifugal distortion coefficients, as well as diagonal and (in some cases) off-diagonal quadrupole tensor components have also been extracted. Diagonalisation of the quadrupole tensors for the ³⁵Cl isotopomers results in a near-cylindrical quadrupole tensor, distorted in the C-Cl-H plane. This distortion, which is somewhat larger than in several comparable systems, supports the assertion that the most prevalent conformations are stabilised by an intramolecular hydrogen bond.     

Microwave spectroscopic study of the CO-H2S complex

Rotational spectra of the CO-H₂S complex were studied using a cavity Fourier transform microwave spectrometer. Altogether 16 isotopomers were investigated. The tunneling splitting due to interchange of the “bonded” and “non-bonded” hydrogen (deuterium) nuclei of the H₂S (D₂S) subunit was observed and analyzed. In addition, the nuclear quadrupole hyperfine structures due to the quadrupolar ³³S and ¹⁷O nuclei could be resolved and analyzed. The resulting rotational, tunneling, and hyperfine constants were used to derive structural and dynamical information about the complex.     

Microwave rotational spectra of the Kr-NH3 van der Waals complex

Rotational spectra of the Kr-NH₃ van der Waals complex were measured in the frequency range between 4 and 24 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. The isotopomers studied included those of NH₃, ¹⁵NH₃, ND₃, NHD₂, and NH₂D with the five most abundant isotopes of Kr. Tunnelling splittings due to the inversion of the ammonia subunit within the ground state of the complex were observed for all three deuterium containing isotopomers. In the NH₃ and ¹⁵NH₃ isotopomers, one of the tunnelling states has a spin statistical weight of zero and the splitting can therefore not be measured in these species. Nuclear quadrupole hyperfine structure arising from the ¹⁴N and ⁸³Kr nuclei was measured and the corresponding nuclear quadrupole coupling constants were determined. These were used to estimate structural parameters and derive information about the intermolecular dynamics. Smaller nuclear quadrupole splittings arising from the deuterium nuclei were observed but could not be resolved. The ground state spectroscopic constants were compared with experimental and theoretical data previously reported for Ar-NH₃ and its isotopomers. For the Kr-ND₃ isotopomers, additional transitions were observed and assigned to the two inversion components of an excited internal rotor state. A fit of the spectroscopic constants revealed the presence of a Coriolis perturbation, similar to that reported for this state in Ar-ND₃ and Ar-NH₃.     

The millimeter wave rotational spectrum of N-cyanomethanimine,CH2NCN

The rotational spectrum of the short-lived species N-cyanomethanimine, CH₂NCN, has been measured in the frequency range 100–250 GHz. The observed transitions allow the determination of the rotational and centrifugal distortion constants and the nitrogen quadrupole coupling constants for both nitrogen nuclei. The N-cyanomethanimine spectrum was measured directly in the products of the pyrolysis of trimethylenetetrazole. The rotational constants obtained are A = 63 372.995(11) MHz, B = 5 449.347 90(28) MHz, and C = 5 009.559 86(29) MHz; the quadrupole coupling constants are χ_aa = 2.057(39) MHz and χ_bb ? χ_cc = ?7.205(21) MHz for the imine nitrogen, and χ_aa = ?3.264(33) MHz and χ_bb ? χ_cc = ?1.630(18) MHz for the cyano-group nitrogen. The accurate constants obtained allow the calculation of the line position and hyperfine structure of any rotational transition appropriate for a radioastronomical search.     

The microwave spectrum and structure of chlorine isocyanate

The microwave spectra of three isotopic species of chlorine isocyanate, ClNCO, have been measured in the frequency region 8–37 GHz. Spectra have been observed for molecules in both the ground and excited vibrational states, and have yielded values for the rotational constants, inertial defects, centrifugal distortion constants, and nuclear quadrupole coupling constants for both chlorine and nitrogen. The molecule has been shown to be planar, with the following internuclear parameters: fx547-1 < (NCO) = 171° 24′ ± 1° 30′, with Cl and O trans. The principal values of the chlorine nuclear quadrupole coupling tensor were calculated, and were found to be consistent with the derived structure.     

Microwave and submillimeterwave spectra of CH2DI and CHD2I

The pure rotational spectra of CH₂DI and CHD₂I have been measured by microwave Fourier transform spectroscopy, millimeterwave spectroscopy and submillimeterwave spectroscopy. The quadrupole hyperfine structure due to iodine has been analyzed by direct diagonalization of the quadrupole tensor. For the J = 1-0 transition of the ground state of CH₂DI, the quadrupole hyperfine structure due to deuterium could be resolved and the quadrupole coupling constant eQq_aa(D) determined.Accurate rotational and centrifugal distortion constants (up to sextic terms) have been determined. They are compared to the constants derived from the ground state combination differences (GSCD). A good agreement is observed but it is also found that the two kinds of data (GSCD and microwave) are complementary and a combined fit allows us to significantly improve the accuracy of the constants.     

The Pure Rotational Spectrum, Geometry, and Hyperfine Constants of Hafnium Monosulfide, HfS

The pure rotational spectra of seven isotopomers of hafnium monosulfide have been measured for several vibrational states. For the most abundant isotopomer, ¹⁸⁰Hf³²S, the J=1 - 0, J=2 - 1, and J=3 - 2 transitions were recorded up to the sixth vibrationally excited state. The constants Y₀₁, Y₀₂, Y₁₁, Y₂₁, and Y₃₁ were determined via a multi-isotopomer fit to a Dunham-type expression. In the process of fitting the data it was necessary to include Born-Oppenheimer breakdown correction terms. The equilibrium internuclear distance has been evaluated. For both the ¹⁷⁷Hf³²S and ¹⁷⁹Hf³²S isotopomers, nuclear hyperfine structure due to the hafnium nucleus was observed and notably large Hf nuclear quadrupole coupling constants, eQq, were determined.     

High-Resolution IR Spectroscopy of the N2O-H2O and N2O-D2O van der Waals Complexes

We report high-resolution infrared vibrational-rotational spectra of the weakly bound complexes N₂O-H₂O and N₂O-D₂O in the higher frequency N₂O stretching mode region (ν₃=2223.756693(124) cm⁻¹). The measurements were carried out using a free jet expansion in combination with a lead salt diode laser spectrometer. Rotational constants, quartic centrifugal distortion constants, and band origins have been derived for both isotopomers. The geometrical structure is determined using isotopic substitution. The deduced structure shows evidence for a second hydrogen bond interaction within the complex. The nonrigidity of the complexes gives rise to an internal rotation of the water molecule around its own C_2v symmetry axis. For N₂O-H₂O, a tunneling splitting arising from this internal motion has been observed in the spectra. According to symmetry considerations, the observed splitting in the spectrum of N₂O-H₂O corresponds to the difference between the tunneling frequencies in the ground and vibrationally excited states.