Nuclear Spin-rotation Interaction Research Articles

Parity-violating contributions to nuclear spin-rotation interactions and to NMR shielding constants in tetrahedral molecules

In natural processes involving weak interactions, a violation of spatial parity conservation should appear. Although the parity-violation effects are expected to be observable in molecular systems, their tiny magnitude has prevented their detection to date. We present a theoretical analysis of four-component relativistic nuclear-spin-dependent parity-violating nuclear spin-rotation and NMR shielding tensors in a set of tetrahedral chiral molecules. This work emphasizes the significant contribution of the ligands and the electronic structure of the chiral center in enhancing these effects, paving the way for the targeted design of promising molecules for measurements.

A variational model for the hyperfine resolved spectrum of VO in its ground electronic state.

A variational model for the infra-red spectrum of vanadium monoxide (VO) is presented, which aims to accurately predict the hyperfine structure within the VO XΣ-4 electronic ground state. To give the correct electron spin splitting of the XΣ-4 state, electron spin dipolar interaction within the ground state and the spin-orbit coupling between XΣ-4 and two excited states, AΠ4 and 1Σ+2, are calculated ab initio alongside hyperfine interaction terms. Four hyperfine coupling terms are explicitly considered: Fermi-contact interaction, electron spin-nuclear spin dipolar interaction, nuclear spin-rotation interaction, and nuclear electric quadrupole interaction. These terms are included as part of a full variational solution of the nuclear-motion Schrödinger equation performed using program Duo, which is used to generate both hyperfine-resolved energy levels and spectra. To improve the accuracy of the model, ab initio curves are subject to small shifts. The energy levels generated by this model show good agreement with the recently derived empirical term values. This and other comparisons validate both our model and the recently developed hyperfine modules in Duo.

A Method for the Variational Calculation of Hyperfine-Resolved Rovibronic Spectra of Diatomic Molecules.

An algorithm for the calculation of hyperfine structure and spectra of diatomic molecules based on the variational nuclear motion is presented. The hyperfine coupling terms considered are Fermi-contact, nuclear spin-electron spin dipole–dipole, nuclear spin–orbit, nuclear spin-rotation, and nuclear electric quadrupole interactions. Initial hyperfine-unresolved wave functions are obtained for a given set of potential energy curves and associated couplings by a variation solution of the nuclear-motion Schrödinger equation. Fully hyperfine-resolved parity-conserved rovibronic Hamiltonian matrices for a given final angular momentum, F, are constructed and then diagonalized to give hyperfine-resolved energies and wave functions. Electric transition dipole moment curves can then be used to generate a hyperfine-resolved line list by applying rigorous selection rules. The algorithm is implemented in Duo, which is a general program for calculating spectra of diatomic molecules. This approach is tested for NO and MgH, and the results are compared to experiment and shown to be consistent with those given by the well-used effective Hamiltonian code PGOPHER.

Conversion of ortho and para nuclear spin isomers of methanol studied by internal axis method

Quantum statistics of identical nuclei in molecules gives rise to nuclear spin isomers. Except for methanol (CH3OH) that has internal rotation (i.e., torsion), all the studied molecules so far have overall rotational symmetry. Methanol does not rotate around the overall rotational symmetry axis but the methyl group axis. The identical nuclei in the methyl group produce a specific type of nuclear spin isomers of the ortho-CH3OH and para-CH3OH in methanol. Conversions between these two spin isomers have been observed in the experiment and a theory based on the rho-axis method (RAM) for partially explaining their conversions has been developed by us recently. However, besides RAM the internal axis method (IAM) is a classical method commonly used by many researchers and the coupling of the angular momenta between torsion and overall rotation in methanol can be completely separated from each other in the IAM. In this paper, as a series work we present a general theoretical model about conversion of ortho–para methanol using the IAM based on quantum relaxation theory. The expressions of the ortho–para state-mixing strengths induced by the nuclear spin–spin and nuclear spin–rotation interactions have been analytically deduced and numerically calculated. We found that the ortho–para states-mixings and ortho–para conversion can be induced by all these two intramolecular interactions whereas the nuclear spin–rotation interaction plays a major role. The conversion rate at temperature 300 K and pressure 1 Torr summed over eight ortho–para level pairs with gaps less than 2.7 GHz is calculated to be 1.4 × 10−2 s−1, which is close to the measured value of 2.1 (3) × 10−2 s−1, indicating the quantum relaxation is the leading process in the spin isomers conversions of methanol and suggesting our theory to be valid for other gaseous methanol-like molecules having torsion tunneling of symmetrical group. The most probable gateways in methanol for collision-induced population transfers are found to be levels-pairs in the ground-state A–E species of (υtJ, K, p) = (0, 24, 4, 1)−(0, 25, 2, 1) by nuclear spin–spin coupling, (0, 22, 5, p)−(0, 21, 6, −1) and (3, 34, 1, 0)−(3, 35, 0, 1) by nuclear spin–rotation coupling.

Quadrupole coupling in alkali metal amides MNH2 ([formula omitted]A1): An experimental and computational study

Rotational spectra of LiNH2 and NaNH2 have been recorded using Fourier transform microwave/millimeter-wave (FTMmmW) techniques in the range 22 – 59 GHz. The species were created from the reaction of metal vapor and ammonia, diluted in argon, using a Discharge-Assisted Laser Ablation Source (DALAS). The JKa,Kc = 101 → 000 transition was measured for both molecules, as well as the JKa,Kc = 202 → 101 transition for NaNH2, all of which exhibited quadrupole coupling splittings. The two data sets were each analyzed with an S-reduced asymmetric top Hamiltonian, establishing the lithium and sodium electric quadrupole coupling parameter, χaa for the first time, and refining previous rotational constants. Quadrupole and nuclear-spin rotation interactions were also computationally investigated at the MP2/6-311G++(3df,2pd) level for LiNH2, NaNH2 and KNH2. These calculations suggest that the major contributor to the quadrupole interactions is the alkali metal nucleus, not that of nitrogen, as confirmed experimentally. Comparison of quadrupole coupling constants suggest that LiNH2 and NaNH2 are principally ionic molecules with a charge distribution similar to LiF and NaF.

Fourier transform microwave spectroscopy of the SiCl+ ion

Fourier transform microwave spectra for the J = 1 ← 0 and 2 ← 1 rotational transitions of the SiCl+ ion were observed for two isotopologues (35Cl and 37Cl) in the ground and the first excited vibrational states of the ground 1Σ+ electronic state. Thanks to the high resolution of the FTMW spectrometer, hyperfine structures due to the quadrupole moment of the chlorine nucleus and the nuclear spin-rotation interaction were fully resolved. The observed FTMW spectra were combined with previously reported MMW and diode laser spectra in an analysis to determine the mass-independent Dunham coefficients Uk,l as well as a mass scaling parameter Δ01Cl=-0.856(30). The equilibrium bond length of SiCl+ determined is re=1.9439729(10) Å and the nuclear quadrupole coupling constant of Si35Cl+ is eQqe=-11.8788(23) MHz.

The pure rotational spectrum of ruthenium monocarbide, RuC, and relativistic ab initio predictions

The J = 1 ← J = 0 and J = 2 ← J = 1 rotational transitions of ruthenium monocarbide, RuC, have been recorded using the separated field pump/probe microwave optical double resonance technique and analyzed to determine the fine and hyperfine parameters for the X(1)Σ(+) state. The (101)Ru(I = 5/2) electric quadrupole parameter, eq0Q, and nuclear spin-rotation interaction parameter, C(I)(eff), were determined to be 433.19(8) MHz and -0.049(6) MHz, respectively. The equilibrium bond distance, r(e), was determined to be 1.605485(2) Å. Hartree-Fock and coupled-cluster calculations were carried out for the properties of the X(1)Σ(+) state. Electron-correlation effects are pronounced for all properties studied. It is shown that (a) the moderate scalar-relativistic contribution to eq0Q is entirely due to the coupling between scalar-relativistic and electron-correlation effects, (b) the spin-free exact two-component theory in its one-electron variant offers a reliable and efficient treatment of scalar-relativistic effects, and (c) non-relativistic theory performs quite well for the prediction of C(I)(elec), provided that electron correlation is treated accurately.

Investigation of parity violation in nuclear spin-rotation interaction of fluorooxirane

When it comes to the question of how signatures from parity violation in chiral molecules can be revealed, then ultra-high resolution spectroscopy techniques play a crucial role in attempts to detect such effects via line shifts. A prerequisite for this approach towards molecular parity violation is a detailed understanding of the corresponding conventional spectroscopy, including even small effects that are often neglected. Nuclear spin-rotation (NSR) interaction is one such effect that becomes appreciable only at high to ultra-high resolution and is often masked by nuclear electric quadrupole interactions (NQI) if nuclei are present with nuclear spin quantum number I larger than 1/2. Herein, NSR interactions are studied theoretically for the chiral molecules fluorooxirane and bromochlorofluoromethane (CHBrClF). For the latter molecule, NSR interactions are compared to nuclear electric quadrupole coupling contributions. The former molecule is devoid of I > 1/2 nuclei such that NSR interactions are not masked by NQI. Parity-odd contributions to NSR of 19F are computed that give rise to additional line shifts in hyperfine transitions. A detection of these nuclear spin-dependent P-odd terms could in principle give information on the nuclear anapole moment in fluorine, which arises due to parity violating interactions within the nucleus.

Frame Transformation Relations and Symmetry Analysis of Fluxional Symmetric Rotor Dimers

The theory of Frame transformation relations between the states of Born Oppenheimer and the weak coupling approximations is developed for polyatomic molecules. The symmetry relations are a generalization of the frame transformation relations derived by Harter and Crogman for coupled rotor molecules. A key internal symmetry label (named “soul”) is defined so that it remains a constant label for frame transformation relations, and is conserved during vibronic transitions, ionization, and even dissociation provided the nuclear spin-rotation interaction is relatively small. Simplified procedures are given for obtaining selection rules, statistical weights, and matrix elements of multipole operators for common molecules having various point symmetries.

The pure rotational spectrum of platinum monocarbide, PtC

The J=1←J=0 and J=2←J=1 rotational transitions of platinum monocarbide, PtC, have been recorded using the separated field pump/probe microwave optical double resonance technique and analyzed to determine the fine and hyperfine parameters for the X1Σ+(v=0) state. The 195Pt (I=1/2) nuclear spin–rotation interaction parameter, CIeff, was determined to be +0.138(6)MHz. A comparison with other Pt-containing molecules and a proposed molecular orbital correlation diagram is given.

S119nspin-lattice relaxation inα-SnF2

The temperature and magnetic field dependencies of the $^{119}\text{S}\text{n}$ nuclear spin-lattice relaxation rate in $\ensuremath{\alpha}{\text{-SnF}}_{2}$ indicate the presence of two relaxation mechanisms. At temperatures below 350 K, the relaxation is dominated by a nuclear spin-rotation interaction modulated by lattice vibrations, as has been seen for Pb and Tl salts. This $^{119}\text{S}\text{n}$ relaxation pathway is less effective in ${\text{SnF}}_{2}$ than it is for $^{207}\text{P}\text{b}$, $^{203}\text{T}\text{l}$, and $^{205}\text{T}\text{l}$ relaxation in some Pb and Tl salts but it is more effective than $^{111}\text{C}\text{d}$ and $^{113}\text{C}\text{d}$ relaxation in some Cd salts. Above 350 K, there is an additional contribution to the observed relaxation rate. The most likely candidate for this thermally activated contribution is the modulation of the $^{119}\text{S}\text{n-}^{19}\text{F}$ dipolar interaction by fluoride-ion motion.

Millimeter-wave spectroscopy of CoNO Produced by UV laser photolysis of Co(CO)3NO

The rotational spectrum of cobalt mononitrosyl (CoNO) produced by ultraviolet photolysis of Co(CO)(3)NO was observed in the millimeter-wave region. Seven rotational transitions in the ground state ranging from J = 6-5 to 12-11, with hyperfine splittings due to the Co nucleus (I = 7/2), were detected in a supersonic jet environment, while higher-frequency transitions in the range from J = 29-28 to 35-34 were measured in the ground, nu(1), nu(2), nu(3), and 2nu(2) vibrational states using a free-space absorption cell. It was confirmed from the observed spectral pattern that the CoNO molecule has a linear structure with the electronic ground state of (1)Sigma(+) symmetry. The rotational lines in the 2nu(2)(Sigma) and nu(3) states were observed to be perturbed by Fermi resonance. The equilibrium rotational constant B(e) is determined to be 4682.207(15) MHz. The CoN bond length is derived to be 1.5842 A assuming the NO bond length of 1.1823 A. A large nuclear spin-rotation interaction constant, C(I) = 123.8(11) kHz, was determined, suggesting a (1)Pi electronic excited state lying close to the ground state.

Analysis of the nuclear quadrupole interaction of Disulfur Dichloride, S 2Cl 2

Pure rotational spectra of S 2 35Cl 2 and S 2 35Cl 37Cl have been observed using a Fourier-transform microwave spectrometer. An analysis of the hyperfine structure made by considering the nuclear spin statistics showed that S 2Cl 2 has C 2 symmetry, where the hyperfine splittings due to the two Cl nuclei were analyzed precisely. The nuclear quadrupole coupling constants including the off-diagonal ( χ ab, χ ac, χ bc) components and the nuclear spin–rotation interaction constants associated with the two Cl nuclei have been determined for the first time. We have shown that the nuclear quadrupole interaction plays an important role in the ortho– para mixing.

The hyperfine structure of sugars investigated by microwave spectroscopy and quantum chemical calculations

The hyperfine structure of glycolaldehyde and 1,3-dihydroxyacetone has been observed using high-resolution microwave Fourier transform spectroscopy. The nuclear spin–rotation interaction was evaluated ab initio at the B3LYP/aug-cc-pVTZ level of the theory using the B3LYP/6-311++G(d, p) molecular structure, and was found to be negligible. Using a suitable Hamiltonian model, the nuclear spin–spin coupling tensor was determined from the molecular structure and from the experimental spectrum. The model developed for the smallest aldose and ketose can be extended to any sugar.

The hyperfine structure of 129 I 2 and 127 I 129 in the B band system

In a double saturation spectroscopy experiment, using the radiation from one single frequency cw laser, the spectra of 129I2 and 127I129I from one saturation setup were recorded simultaneously and with respect to the spectrum of 127I2 from another setup while tuning the laser. The hyperfine patterns of 129I2 and 127I129I were analysed to determine the nuclear electric quadrupole interaction and the nuclear spin–rotation interaction parameters. Models are presented describing the observations and giving reliable predictions for the hyperfine parameters of all isotopomers.

Can ortho-para transitions for water be observed?

The spectrum of water can be considered as the juxtaposition of the spectra of two molecules, with different total nuclear spin: ortho-H2O, and para-H2O. No transitions have ever been observed between the two different nuclear-spin isotopomers. The interconversion time is unknown and it is widely assumed that interconversion is forbidden without some other intervention. However, weak nuclear spin-rotation interaction occurs and can drive ortho to para transitions. Ab initio calculations show that the hyperfine nuclear spin-rotational coupling constants are about 30 kHz. These constants are used to explore the whole vibration-rotation spectrum with special emphasis on the coupling between nearby levels. Predictions are made for different spectral regions where the strongest transitions between ortho and para levels of water could be experimentally observed.

Anisotropic nuclear spin relaxation in single-crystal xenon

We extend the theory of longitudinal spin relaxation of ${}^{129}\mathrm{Xe}$ nuclei in frozen xenon to the case of single-crystal samples, where the relaxation rate depends on the direction of the applied magnetic field with respect to the crystalline axes. For sufficiently large magnetic fields, the relaxation is dominated by spin-flip Raman scattering of lattice phonons. Two closely related interactions couple the lattice phonons to the spins of ${}^{129}\mathrm{Xe}$ nuclei: the nuclear spin-rotation interaction between nearest-neighbor atoms, which leads to an isotropic, field-independent relaxation rate, and the paramagnetic antishielding of the externally applied field at the site of ${}^{129}\mathrm{Xe}$ nuclei by the electrons of neighboring Xe atoms. The latter interaction, also known as the chemical shift anisotropy (CSA) interaction, leads to an anisotropic relaxation rate proportional to the square of the applied field. This mechanism dominates spin relaxation at fields of the order of the Debye field ${B}_{D}{=k}_{B}{T}_{D}/{\ensuremath{\mu}}_{B}=82 \mathrm{T}.$

Fourier transform millimeter-wave spectroscopy of chlorocarbene (HCCl)

The 101–000 and 202–101 rotational transitions of HC35Cl and HC37Cl in the X̃ 1A′ ground vibronic state have been observed with a Fourier transform millimeter-wave spectrometer. The HCCl molecule is produced by discharging a gaseous sample of CH2Cl2 diluted in Ar with a pulsed discharge nozzle. The effective rotational constant (B+C)/2, the centrifugal distortion constant ΔJ, the nuclear quadrupole interaction constants, and the nuclear-spin rotation interaction constant are determined for each isotopic species. The nuclear-spin rotation interaction is found to make a significant contribution to the hyperfine structure of this molecule, which originates from the relatively low-lying electronic excited state. The nuclear quadrupole interaction tensor is highly asymmetric, indicating a significant π character of the C–Cl bond. This can be interpreted in terms of the backdonation of π electrons from the chlorine atom to the carbon atom.

The rotational spectrum of dichlorocarbene, C35Cl2, observed by molecular beam-Fourier transform microwave spectroscopy

The rotational spectrum of the most abundant isotopomer of dichlorocarbene, C35Cl2, in its 1A1 ground electronic state has been recorded using a molecular beam—Fourier transform microwave spectrometer. The CCl2 was generated by flash pyrolysis in a pulsed free jet expansion using CBr2Cl2 or CHCl2COCl as precursors. 43 hyperfine structure lines of 6 rotational transitions were recorded in the frequency range 8–40 GHz. The analysis of the observed spectrum yielded the rotational constants, the complete 35Cl nuclear quadrupole coupling tensor, and the nuclear spin-rotation interaction constants. In addition, a refined r0 structure of CCl2 was derived.

Nuclear spin–rotation interaction in CF 2 ( [formula omitted]) observed by Fourier transform microwave spectroscopy

The rotational spectrum of difluoromethylene (CF 2) in its X ̃ 1A 1 ground electronic state has been observed using molecular beam–Fourier transform microwave spectroscopy. CF 2 was produced in a free jet expansion in Ar as inert carrier gas by ArF excimer laser photolysis of CF 2Cl 2, CF 2CHCl, or CF 2CHBr or by flash pyrolysis of CF 2Br 2 in the high-pressure region directly behind a pulsed molecular beam valve. Rotational transitions were observed in the frequency range from 8 to 40 GHz in the CF 2 (000) ground vibrational state and in the (100), (010), and (001) excited vibrational states. Several observed lines were split due to the hyperfine interaction between the 19 F nuclear spin and the overall rotation of the molecule. The results were used to determine the nuclear spin–rotation interaction parameters for CF 2.