Nuclear Quadrupole Coupling Tensor Research Articles

Weak hydrogen bonding to halogens and chirality communication in propanols: Raman and microwave spectroscopy benchmark theory.

Constitutional and conformational isomers of bromopropanol are vibrationally and rotationally characterised with parallels drawn to the structural chlorine analogues. A previous microwave spectroscopic study of the chloropropanols is re-examined and all systems are explored by Raman jet spectroscopy. For bromine, the entire nuclear quadrupole coupling tensors are accurately determined and compared to their chlorine counterparts. Tensor asymmetry parameters are determined and linked with the hydrogen bond strength as indicated by the downshift of the OH-stretching frequency. The spectroscopic constants derived from the observed transitions are used as benchmarks for a large variety of electronic structure methods followed by harmonic and anharmonic rovibrational treatments. The CCSD(T) electronic structure calculations provide the best performance, in particular once anharmonic and relativistic corrections are applied or implied. Standard DFT approaches vary substantially with respect to their systematic error cancellation across the investigated species, and cost-effective compromises for the different observables are proposed.

Partial Proton Transfer in the Gas Phase: A Spectroscopic and Computational Analysis of the Trifluoroacetic Acid - Trimethylamine Complex.

The 1:1 complex formed from trifluoroacetic acid (TFA) and trimethylamine (TMA) has been observed in the gas phase by rotational spectroscopy and further investigated by DFT and MP2 methods. Spectra of both the parent form and the -OD isotopologue have been obtained. The complex is structurally similar to a hydrogen bonded system, with the O-H bond directed toward the nitrogen of the TMA. However, both the spectroscopic and computational results indicate that it is intermediate between a hydrogen bonded complex and a proton-transferred ion pair. Two metrics are used to assess the degree of proton transfer from the acid to the base. The first is based on experimental 14N nuclear quadrupole coupling constants. Specifically, the component of the 14N nuclear quadrupole coupling tensor along the c-inertial axis of the complex, χcc, is 31% of the way between that of free TMA (no proton transfer) and that of TMAH+ (complete proton transfer). A second metric, adapted from that of Kurnig and Scheiner [Int. J. Quantum Chem. Quantum Biol. Symp. 1987, 14, 47-56], is based on calculated O-H and H-N distances and corroborates this description. These results indicate that the degree of proton transfer in TFA-TMA is very similar to that in the TMA complex of HNO3, which has been previously studied and for which the proton affinity of the conjugate anion (NO3-) is almost identical to that of CF3COO-. While the solid salt, TMAH+·CF3COO-, is an ionic plastic above 307 K and exhibits free rotation of the ions, no such motion is observed in the cold 1:1 gas phase adduct.

A comparative analysis of powder ENDOR spectra of aromatic and aliphatic radicals by exact and 1st order simulation

Exact and perturbation methods were employed in simulations of powder ENDOR spectra to obtain the anisotropic hyperfine ( hfc ) and nuclear quadrupole ( nqc ) coupling constants of certain organic radicals of interest in fundamental research and in applications in radiation research, surface chemistry and biophysics. The principal hfc values of the ring protons and methyl substituents for several aromatic cations trapped in disordered matrices might be more accurate than those previously reported using regular EPR. Only one of the earlier assignments of the naphthalene cation spectrum was in acceptable agreement with the simulations. The proton couplings at the β-position of alkyl radicals were deduced by simulations while the spectrum due to α-couplings with appreciable anisotropy was weak. Accurate simulation of the 14 N (I = 1) spectra in bio-radicals was obtained by adjustment of the relative orientation of the principal hfc and nuclear quadrupole coupling ( nqc ) tensors as well as the principal values. Adjustment of the excitation width parameter employed in the software was also required in a few cases to improve the agreement with the experimental spectra. The hfc patterns due to matrix nuclei ( 1 H and 7 Li) around the radicals of X-irradiated samples were simulated to elucidate the nature of the trapping sites in materials used for EPR dosimetry. ENDOR simulation programs known to us are presented in an Appendix. The performance of the ENDORF2 program used in previous works was examined by comparison with exact treatment. Input and output examples, source and executable codes used in this work can be downloaded at https://github.com/EndorF2/Simulation .

Rotational spectra and conformational analysis of 2-bromobutane

The rotational spectra of 2-bromobutane were obtained by using a chirped-pulse Fourier-transform microwave (CP-FTMW) spectrometer in the frequency range 8–18 GHz. Hyperfine structures of both bromine isotopologues (I = 3/2) for the three conformers (G+, A, and G−) have been investigated to determine the rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants of bromine (Br). The conformational compositions for G+, A, and G− in the supersonic jet, estimated by relative intensity measurements, are 55%, 28%, 17%, respectively, for the 79Br species, and 52%, 33%, 15%, respectively, for the 81Br species. However, the small zero-point energy differences of isotopomers can be negligible and can be considered as an average value instead of individual values, which is 54%, 30%, 16% for G+, A, and G− species. The nuclear quadrupole coupling tensors (χxx, χyy, and χzz) of Br and the quadrupolar angles were obtained, and the χzz value of 2-bromobutane is compared with those of other alkyl bromides. The spectroscopic constants, rs coordinates of Br and the dipole moments of the most stable G+ conformer are predicted well from the ab initio calculations at the MP2/6–311++G(2d,2p) level.

Solid-State Nuclear Magnetic Resonance Techniques for the Structural Characterization of Geminal Alane-Phosphane Frustrated Lewis Pairs and Secondary Adducts.

The first comprehensive solid‐state nuclear magnetic resonance (NMR) characterization of geminal alane‐phosphane frustrated Lewis pairs (Al/P FLPs) is reported. Their relevant NMR parameters (isotropic chemical shifts, direct and indirect 27Al‐31P spin‐spin coupling constants, and 27Al nuclear electric quadrupole coupling tensor components) have been determined by numerical analysis of the experimental NMR line shapes and compared with values computed from the known crystal structures by using density functional theory (DFT) methods. Our work demonstrates that the 31P NMR chemical shifts for the studied Al/P FLPs are very sensitive to slight structural inequivalences. The 27Al NMR central transition signals are spread out over a broad frequency range (>200 kHz), owing to the presence of strong nuclear electric quadrupolar interactions that can be well‐reproduced by the static 27Al wideband uniform rate smooth truncation (WURST) Carr‐Purcell‐Meiboom‐Gill (WCPMG) NMR experiment. 27Al chemical shifts and quadrupole tensor components offer a facile and clear distinction between three‐ and four‐coordinate aluminum environments. For measuring internuclear Al⋅⋅⋅P distances a new resonance‐echo saturation‐pulse double‐resonance (RESPDOR) experiment was developed by using efficient saturation via frequency‐swept WURST pulses. The successful implementation of this widely applicable technique indicates that internuclear Al⋅⋅⋅P distances in these compounds can be measured within a precision of ±0.1 Å.

Analysis of the Complex Quadrupole Hyperfine Patterns for Two Chlorine Nuclei in the Rotational Spectrum of 2,5-Dichlorothiophene.

The rotational spectrum of 2,5-dichlorothiophene (DCT) was measured for the first time using Fourier transform microwave spectroscopy from 5.5-19 GHz. Dense hyperfine splitting patterns due to the two quadrupolar chlorine nuclei (I = 3/2) were resolved and assigned for the 35Cl-35Cl, 37Cl-35Cl, and 37Cl-37Cl isotopologues and for the two 13C and one 34S analogues with two 35Cl atoms, allowing derivation of their respective nuclear quadrupole coupling tensors. The rotational constants obtained from fitting the spectra of the six isotopic species allowed derivation of the experimental geometry of DCT for comparison with the equilibrium structure computed at the MP2/aug-cc-pVTZ level. This revealed that the electron-withdrawing effect of chlorine causes small distortions in the ring geometry relative to thiophene, including a 1.1° increase in the two S-C-C angles and a 0.012 Å increase in the two S-C bond lengths.

Quasi-relativistic study of nuclear electric quadrupole coupling constants in chiral molecules containing heavy elements

A quasi-relativistic (two-component) zeroth-order regular approximation model potential approach is employed to calculate nuclear electric quadrupole coupling tensors of the chiral polyhalomethane derivatives CHBrClF and CHClFI. Each of these two chiral compounds contains two nuclei with electric quadrupole moments, which poses special challenges for an analysis of high-resolution rovibrational spectra and, thus, renders predictions of electric field gradients and resulting nuclear electric quadrupole coupling tensors in such molecules particularly valuable. The theoretical approach is studied with respect to the influence of relativistic effects, by comparison to scalar-relativistic and non-relativistic calculations, and electron correlation effects, which are included within different flavours of density functional theory. The quality of the calculations is assessed by comparison to results from experiments reported previously by Bauder et al. and Soulard et al., respectively. It is shown that the local density approximation exchange-correlation functional gives results that agree with experiment within 12 % for all nuclear quadrupole coupling tensor components of 79Br and 127I. This benchmark test of the quasi-relativistic approach indicates its applicability to support upcoming microwave experiments with chiral heavy-element containing main group compounds.

Dipole forbidden, nuclear electric quadrupole allowed transitions and chirality: The broadband microwave spectrum and structure of 2-bromo-1,1,1,2-tetrafluoroethane

The chirped pulse Fourier transform microwave spectrum of the chiral molecule 2-bromo-1,1,1,2-tetrafluoroethane has been observed in the 6–18 GHz region of the electromagnetic spectrum and is reported. Six isotopologues, including 13C isotopic substitution at each carbon atom in both the 79Br and 81Br parent species have been observed in natural abundance. All rotational constants, quartic centrifugal distortion contants, and nuclear quadrupole coupling tensor components have been experimentally determined and have been reported. In addition, spin-rotation components Maa, Mbb, and Mcc are also determined and reported. The theoretical structure of the species has been determined and reported. The C–C–Br backbone structure has been determined experimentally implementing both Kraitchman single and Rudolph double substitution analysis, the later being explained in the text. 3440 transitions total have been observed with 45 transitions being electric dipole forbidden. These dipole forbidden transitions have been identified as they are planned to be used in microwave three-wave mixing experiments in order to better understand their role, if any, in determining chirality. The determined nuclear electric quadrupole coupling tensor has been diagonalized and compared to similar small alkane species in order to make inferences on the effects of fluorination on the C–Br bond.

Probing resonance effects in aromatic systems by nuclear quadrupole Coupling: Investigations of 3- and 4-chlorophenol by rotational spectroscopy

Rotational transitions of the only conformer of 4-chlorophenol and the two conformers of 3-chlorophenol (syn and anti) were measured in the frequency range of 10.5–21.0 GHz using broadband pulsed-jet Fourier transform microwave spectroscopy. For the two conformers of 3-chlorophenol, the complete nuclear quadrupole coupling tensors of chlorine, χ, were obtained. Rotational, centrifugal distortion and quadrupole coupling constants were determined for each conformational species, comprising the 35Cl and 37Cl isotopologues. An extended Townes-Dailey analysis was performed on the obtained quadrupole coupling tensors and the results are discussed regarding the effects of different functional groups at different substitution positions of the benzene ring. Together with reanalyzed literature data, this analysis supplements established concepts in organic chemistry to predict and explain the reactivity of aromatic molecules.

A Modified Townes-Dailey Model for Interpretation and Visualization of Nuclear Quadrupole Coupling Tensors in Molecules

We propose a modified Townes-Dailey (TD) model to help interpret and visualize experimentally measureable nuclear quadrupole coupling tensors (thus the electric field gradient tensors) in molecules. We show that, within the framework of the TD model, each principal component of the nuclear quadrupole coupling tensor is directly related to a new quantity termed as the valence p-orbital population anisotropy (VPPA or ΔP) in the same direction. While the proposed model is a simple reformulation of the original TD model thus does not introduce new physics, the concept of VPPA makes it possible to directly interpret as well as visualize, in a much straightforward way, the experimentally determined nuclear quadrupole coupling tensors in molecules. We illustrate the utilization of VPPA using nuclear quadrupole coupling tensors for 11B, 14N, 17O, 35Cl, 87Br, and 127I nuclei in a variety of molecules. We propose to use VPPA or ΔP ellipsoid representation as a means of visualizing/displaying nuclear quadrupole coupling tensors in the molecular frame. We show the usefulness of the VPPA concept in providing a unifying explanation for seemingly different types of molecular interactions such as hydrogen bonding, halogen bonding, and frustrated Lewis pairs. We further suggest that VPPA can be used as a universal measure of the ability of any element in the entire p-block of the periodic table (groups 13-16) to interact with nucleophiles (e.g., formation of chalcogen, pnictogen, tetrel, and triel bonds).

Determination and analysis of the nuclear quadrupole coupling tensors of 2-bromopyridine

The rotational spectrum of 2-bromopyridine was reinvestigated in the frequency range of 10–15.5 GHz by Fourier transform microwave (FTMW) spectroscopy. New observations of 14N hyperfine splittings have been observed for both the bromine-79 and -81 isotopologues in addition to five centrifugal distortion constants. Rotational constants and bromine nuclear quadrupole coupling tensors have been refined. Both the bromine and the nitrogen nuclear quadrupole coupling tensors have been investigated using the Extended Townes-Dailey analysis.

Nuclear quadrupole coupling in SiH2I2 due to the presence of two iodine nuclei

The rotational spectrum of diiodosilane was measured with a jet-pulsed, cavity Fourier transform microwave (FTMW) spectrometer over the frequency range 8.8–15GHz and assigned for the first time. The complete nuclear quadrupole coupling (NQC) tensors for both iodine nuclei were obtained for the 28Si, 29Si, and 30Si isotopologues of diiodosilane. In addition to the nuclear quadrupole coupling constants (NQCCs), rotational constants, centrifugal distortion constants, and nuclear spin-rotation constants were determined for each silicon isotopologue. Subtle, yet unmistakable, changes in the NQCCs of iodine upon isotopic substitution were observed and will be discussed. A r0 structure of diiodosilane was also fit via isotopic substitution, which led to the determination of bond lengths and angles: r0(SiI)=2.4236(19)Å, r0(SiH)=1.475(21)Å, ∠(ISiI)=111.27(13)°, and ∠(ISiH)=105.9(19)°. These molecular parameters will be compared to the results of a previous gas electron diffraction study.

Fourier-transform microwave spectroscopy of a halogen substituted Criegee intermediate ClCHOO.

Pure rotational spectra of the chloro-substituted Criegee intermediate (ClCHOO) were observed by Fourier-transform microwave spectroscopy. Two conformers (syn and anti) of the isolated molecule were identified from the rotational spectra of the parent and 37Cl and 13C isotopologues detected in natural abundance. Rotational constants, centrifugal distortion constants, and all components of the nuclear quadrupole coupling tensor were determined for both conformers. Structural features of the molecule have been rationalized with supporting ab initio calculations and the natural bond orbital analysis, which suggest that the conformational preferences are driven by hyperconjugative effects.

Symmetry Breaking Nuclear Quadrupole Coupling Tensor Orientation for Cesium‐133 Nuclei Located in a Mirror Plane

Simultaneous multiple data set fits of all transition peaks of 133Cs nuclei enabled us to obtain accurate cesium‐133 nuclear magnetic resonance (NMR) parameters and Euler angles between the principal axis systems of the chemical shift (CS) and quadrupole coupling (Q) tensors of 133Cs nuclei in Cs2CrO4 . Although in a previous study of Cs2CrO4 by Power et al. (W. P. Power, S. Mooibroek, R. E. Wasylishen, T. S. Cameron, J. Phys. Chem. 1994, 98, 1552), one central transition was observed for cesium sites 1 and 2 in the 133Cs NMR spectra and one Euler angle between the CS tensors and Q tensors was obtained as 52° and 7° for cesium sites 1 and 2, respectively, the present single‐crystal 133Cs NMR measurements found two Euler angles (10(2)°, 51.9(1)°, 0°) for site 1 and two central transition peaks for site 2. Three principal components of the CS tensor for Cs1 are oriented along the crystallographic a, b, and c axes, whereas none of the principal components of the Q tensor for Cs1 are oriented along the crystal axes. The principal component V 22 of the Q tensor for Cs1 is tilted 10° from the b axis in the bc plane, and the other two components are not located in the ac plane. Therefore, we have found that the requirement that “the quadrupole coupling tensor for a nucleus located in a mirror plane has one principal axis perpendicular to the mirror plane” cannot be applied to Cs1. On the other hand, δ11 and V 22 for Cs2 are aligned along the b axis, and the other components of the CS and Q tensors deviate at an angle of 1.4(1)° and 10.1(1)°, respectively, from the a and c axes in the ac plane. A distortion‐free powder 133Cs NMR spectrum of Cs2CrO4 was measured using a solid‐state spin echo technique.

A Study of 2-Iodobutane by Rotational Spectroscopy.

Rotational transitions belonging to 2-iodobutane (sec-butyl-iodide, CH3CHICH2CH3) were measured over the frequency range 5.5-16.5 GHz via jet-pulsed Fourier transform microwave spectroscopy. The complete nuclear quadrupole coupling tensor of iodine, χ, was obtained for the gauche (g)-, anti (a)-, and gauche' (g')-conformers as well as the four (13)C isotopologues of the gauche species. Rotational constants, centrifugal distortion constants, quadrupole coupling constants, and nuclear spin-rotation constants were determined for each species. Changes in χ of the iodine nucleus, resulting from conformational and isotopic differences, are discussed. Isotopic substitution of g-2-iodobutane allowed for an rs structure to be determined for the carbon backbone. Additionally, isotopic substitution in conjunction with an ab initio structure allowed for a fit of various r0 structural parameters belonging to g-2-iodobutane.

The CP-FTMW spectrum of bromoperfluoroacetone

The microwave spectrum of bromoperfluoroacetone has been observed and reported for the first time on a newly constructed CP-FTMW spectrometer located at Missouri S&T operational in the 6–18GHz region of the electromagnetic spectrum. Rotational constants, centrifugal distortion constants, and all components of the nuclear quadrupole coupling tensor were determined for both the 79 and 81 isotopologues of bromine and have been reported. The spectrum is densely populated with transitions and the need for the more accurate theory of W.C. Bailey was needed for adequate assignment of >2000 transitions. Included in the spectrum were dipole forbidden ΔJ=2 and x-type transitions. Second moments and Kraitchman position of the bromine have been presented to help identify the experimental structure. Quadrupole coupling tensor analyses were carried out to show that the electric field gradient in bromine changes dramatically upon fluorination when compared to its chlorine counterpart. Also, this change represents an increase in the electric field gradient, suggesting more covalent C-Br bonding. Dipole forbidden transitions have been observed and found to be always linked through a dipole allowed transition, but are not necessarily linked through one singular off-diagonal term. Notably, a select few of the dipole forbidden transitions are facilitated through a third mixing state not previously found in the literature.

Fourier transform microwave and millimeter-wave spectroscopy of bromoiodomethane, CH2BrI.

Bromoiodomethane, CH2BrI, is a molecule of natural origin emitted in significant amount into the marine boundary layer. It can easily be decomposed by solar radiation, releasing Br and I atoms in the troposphere, which in turn impacts the atmospheric chemistry. Spectroscopy is an invaluable tool to monitor species present in the atmosphere. Since no high-resolution spectroscopic studies are available for this dihalomethane, we have investigated the rotational spectra of the two bromine isotopologues of CH2BrI in its vibrational ground state in the microwave and millimeter-wave regions. Transitions of b-type have been recorded by Fourier transform microwave spectroscopy below 25 GHz while both a- and b-type spectral lines have been measured below 230 GHz. Observed transitions correspond to energy levels with J ≤ 132 and Ka ≤ 14. Molecular constants including those describing the nuclear quadrupole coupling tensors for (79)Br, (81)Br, and (127)I were accurately determined from the least-squares analysis of a total of 1873 distinct transition frequencies (of which 943 belong to the CH2(79)BrI isotopologue). An experimental (r0) structure of the title species has been derived from the two sets of rotational constants.

On ammonia binding to the oxygen-evolving complex of photosystem II: a quantum chemical study.

A recent EPR study (M. Perrez Navarro et al., Proc. Natl. Acad. Sci. 2013, 110, 15561) provided evidence that ammonia binding to the oxygen-evolving complex (OEC) of photosystem II in its S2 state takes place at a terminal-water binding position (W1) on the "dangler" manganese center MnA. This contradicted earlier interpretations of (14)N electron-spin-echo envelope modulation (ESEEM) and extended X-ray absorption fine-structure (EXAFS) data, which were taken to indicate replacement of a bridging oxo ligand by an NH2 unit. Here we have used systematic broken-symmetry density functional theory calculations on large (ca. 200 atom) model clusters of an extensive variety of substitution patterns and core geometries to examine these contradictory pieces of evidence. Computed relative energies clearly favor the terminal substitution pattern over bridging-ligand arrangements (by about 20-30 kcal mol(-1)) and support W1 as the preferred binding site. Computed (14)N EPR nuclear-quadrupole coupling tensors confirm previous assumptions that the appreciable asymmetry may be accounted for by strong, asymmetric hydrogen bonding to the bound terminal NH3 ligand (mainly by Asp61). Indeed, bridging NH2 substitution would lead to exaggerated asymmetries. Although our computed structures confirm that the reported elongation of an Mn-Mn distance by about 0.15 Å inferred from EXAFS experiments may only be reproduced by bridging NH2 substitution, it seems possible that the underlying EXAFS data were skewed by problems due to radiation damage. Overall, the present data clearly support the suggested terminal NH3 coordination at the W1 site. The finding is significant for the proposed mechanistic scenarios of OEC catalysis, as this is not a water substrate site, and effects of this ammonia binding on catalysis thus must be due to more indirect influences on the likely substrate binding site at the O5 bridging-oxygen position.

Theoretical 14N and 17O nuclear quadrupole resonance parameters for tirapazamine and related metabolites

Density functional theory (DFT) calculations were performed to realize the effects of the N–O group on the reactivity and electronic properties of 3-amino-1,2,4-benzotriazines. The electric field gradient, EFG, tensors of 14N and 17O nuclei and natural bond orbital (NBO) analysis in the tirapazamine (TPZ) and its four derivatives were calculated at the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) method in the gas phase. The NBO analysis reveal that the bond strength, proton affinity and position of N–O group in the heterocyclic ring have major influence on the reactivity of considered molecules. Accordingly, we suggest that the TPZ and 4-oxide (d) structures due to having a weaker N–O bond with larger negative charge on the oxygen atom at the 4-position are more active than the other ones. Calculated 14N and 17O EFG tensors were used to evaluate nuclear quadrupole coupling tensors, χ, and asymmetry parameters, η Q . Results showed that oxidation of a nitrogen atom at any position have significant influence on its 14N nuclear quadrupole resonance (NQR) parameters. Also, the occupancy of nitrogen lone pair plays an important role in determination of the q zz and χ values of mentioned nuclei. It is found that the η Q and χ are appropriate parameters to study the contribution of lone pair electrons of nitrogen atom in the formation of chemical bond or conjugation with the aromatic system. Finally, a linear correlation is observed between the χ(14N) and χ(17O) values in the N–O bond which may be associated with the reactivity of these compounds.

Accurate Semiexperimental Structure of 1,3,4-Oxadiazole by the Mixed Estimation Method

In order to determine an accurate equilibrium structure for 1,3,4-oxadiazole, microwave transitions and ground-state rotational constants are reported for the parent species and for the (18)O isotopologue measured in natural abundance. These rotational constants along with those of the (13)C, (15)N, and D1 species were used together with vibration-rotation constants calculated from a cubic force field calculated at the B3LYP/6-311+G(3df,2pd) level of theory to derive a semiexperimental equilibrium structure. However, the results of this fit were not satisfactory; therefore, the structure was later significantly improved by the mixed estimation method. In this method, internal coordinates from good-quality quantum chemical calculations (with appropriate uncertainties) are fitted simultaneously with moments of inertia of the full set of isotopologues. The accuracy of this structure has been confirmed by using an extrapolation technique. All elements of the (14)N nuclear quadrupole coupling tensor have been determined.