Hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2)

The hyperfine structures of the 2 (3)Sigma(g) (+), 3 (3)Sigma(g) (+), and 4 (3)Sigma(g) (+) states of Na(2) have been resolved with sub-Doppler continuous wave perturbation facilitated optical-optical double resonance spectroscopy via A (1)Sigma(u) (+) approximately b (3)Pi(u) mixed intermediate levels. The hyperfine patterns of these three states are similar. The hyperfine splittings of the low rotational levels are all very close to the case b(betaS) limit. As the rotational quantum number increases, the hyperfine splittings become more complicated and the coupling cases become intermediate between cases b(betaS) and b(beta J) due to spin-rotation interaction. We present a detailed analysis of the hyperfine structures of these three (3)Sigma(g) (+) states, employing both case b(betaS) and b(beta J) coupling basis sets. The results show that the hyperfine splittings of the (3)Sigma(g) (+) states are mainly due to the Fermi-contact interaction. The Fermi contact constants for the two d sigma Rydberg states, the 2 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+), are 245+/-5 MHz and 225+/-5 MHz, respectively, while the Fermi contact constant of the s sigma 3 (3)Sigma(g) (+) Rydberg state is 210+/-5 MHz. The diagonal spin-spin and spin-rotation constants, and nuclear spin-electronic spin dipolar interaction parameters of the 3 (3)Sigma(g) (+) and 4 (3)Sigma(g) (+) states are also obtained.     

Nuclear quadrupole hyperfine structure in the microwave spectrum of HCl-N2O: electric field gradient perturbation of N2O by HCl

The microwave spectra of six isotopomers of HCl-N(2)O have been obtained in the 7-19 GHz region with a pulsed molecular beam, Fourier transform microwave spectrometer. The nuclear quadrupole hyperfine structure due to all quadrupolar nuclei is resolved and the spectra are analyzed using the Watson S-reduced Hamiltonian with the inclusion of nuclear quadrupole coupling interactions. The spectroscopic constants determined include rotational constants, quartic and sextic centrifugal distortion constants, and nuclear quadrupole coupling constants for each quadrupolar nucleus. Due to correlations of the structural parameters, the effective structure of the complex cannot be obtained by fitting to the spectroscopic constants of the six isotopomers. Instead, the parameters for each isotopomer are calculated from the A and C rotational constants and the chlorine nuclear quadrupole coupling constant along the a-axis, chi(aa). There are two possible structures; the one in which hydrogen of HCl interacts with the more electronegative oxygen of N(2)O is taken to represent the complex. The two subunits are approximately slipped parallel. For H (35)Cl-(14)N(2)O, the distance between the central nitrogen and chlorine is 3.5153 A and the N(2)O and HCl subunits form angles of 72.30 degrees and 119.44 degrees with this N-Cl axis, respectively. The chlorine and oxygen atoms occupy the opposite, obtuse vertices of the quadrilateral formed by O, central N, Cl, and H. Nuclear quadrupole coupling constants show that while the electric field gradient of the HCl subunit remains essentially unchanged upon complexation, there is electronic rearrangement about the two nitrogen nuclei in N(2)O.     

Rotational spectra of the Xe-(H2O)2 van der Waals trimer: xenon as a probe of electronic structure and dynamics

Rotational spectra of three isotopomers of the Xe-(H2O)2 van der Waals trimer were recorded using a pulsed-nozzle, Fourier transform microwave spectrometer. Nine [nine, four] a-type and twelve [eleven, seven] b-type transitions were measured for the 132Xe-(H2O)2 [129Xe-(H2O)2, 131Xe-(H2O)2] isotopomer. The determined rotational and centrifugal distortion constants were used to extract information about the structure and vibrational motions of the complex. The nuclear quadrupole hyperfine structures due to the 131Xe (nuclear spin quantum number I=3/2) nucleus were also detected. The large value of the off-diagonal nuclear quadrupole coupling constant chiab in particular provides detailed insight into the electronic environment of the xenon atom and the orientations of the water molecules within the complex. An effective structure that best reproduces the experimental 131Xe nuclear quadrupole coupling constants is rationalized by ab initio calculations. An overall goal of this line of work is to determine how the successive solvation of a xenon atom with water molecules affects the xenon electron distribution and its intermolecular interactions. The results may provide molecular level interpretations of 129Xe NMR data from, for example, imaging experiments.     

Gadolinium (III) ion in liquid water: structure, dynamics, and magnetic interactions from first principles

We applied first principles molecular dynamics (MD) technique to study structure, dynamics, and magnetic interactions of the Gd(3+) aqua ion dissolved in liquid water, a prototypical system for Gd-based complexes used as contrast agents for magnetic resonance imaging. The first coordination sphere contains eight water molecules with an average Gd-O distance of 2.37 A and an average geometric arrangement close to a square antiprism. The mean tilt angle of the electric dipole vector of these water molecules is theta=145 degrees . In our picosecond time scale simulation we observe no exchange event from the first coordination sphere but only fast "wagging" motions. The second coordination sphere is well pronounced though water molecules in this sphere are subjected to large amplitude dynamic motions. The isotropic hyperfine coupling constants for the inner sphere water molecules [A(iso)((17)O(I))=0.65+/-0.03 MHz, A(iso)((1)H(I))=0.085+/-0.005 MHz] are in good agreement with experimental data and with an earlier study using classical MD. Second sphere Fermi contact hyperfine coupling constants calculated are more than one order of magnitude smaller and of opposite sign as those of the first coordination sphere. The effect of spin polarization induced by the paramagnetic Gd(3+) ion on the dipolar hyperfine interaction was found to be sizable only for the (17)O nuclei of inner sphere water molecules and has a screening character.     

The hyperfine interaction in the A 2Pi1/2 and X 2Sigma+ states of ytterbium monofluoride

The fine and hyperfine interaction parameters in the A (2)Pi(1/2)(v=0) and X (2)Sigma(+)(v=0) states of the odd metal nuclear spin isotopologues of ytterbium monofluoride, (171)YbF and (173)YbF, have been determined from an analysis of high-resolution laser induced fluorescence spectra of the A (2)Pi(12)<--X (2)Sigma(+)(0,0) band. The observed ground X (2)Sigma(+)(v=0) state (171)Yb(I=1/2) Fermi contact parameter is significantly smaller than that determined from the matrix isolation electron spin resonance measurement [Van Zee et al., J. Phys. Chem. 82, 1192 (1978)]. An interpretation of the (173,171)Yb magnetic hyperfine and nuclear electric quadrupole coupling parameters is given.     

Observation of the 3(3)Sigma(+)-X1Sigma+ transition of KRb by resonance enhanced two-photon ionization in a pulsed molecular beam: Hyperfine structures of 39K85Rb and 39K87Rb isotopomers

The 3(3)Sigma(+)-X1Sigma+ transition of KRb is observed by resonance enhanced two-photon ionization in a pulsed molecular beam. Hyperfine splittings of 39K85Rb and 39K87Rb isotopomers are observed. From the magnitude of hyperfine splittings, we found that the main hyperfine structure was dominated by the Fermi contact interaction between the Rb nuclear spin and the unpaired electron spin. The Fermi contact interaction constants were determined to be 291 MHz for 39K85Rb and 665 MHz for 39K87Rb. In the KRb 3(3)Sigma+ state the electron spin couples more strongly with the Rb nuclear spin than with other angular momenta, and the energy level structure is well described by the hyperfine angular momentum coupling scheme of the b(betaS) case. The molecular constants and the Rydberg-Klein-Rees potential energy curve of the 3(3)Sigma+ state were determined.     

Molecular integrals of relativistic effects with gaussian-type orbitals

A mathematical analysis is presented of molecular integrals of relativistic interactions in molecules. The integrals are based on Gaussian-type orbitals and include those arising from variation of electron mass with velocity, one-electron Fermi contact interaction, electron spin-same-orbit interaction, electron spin-nuclear spin interaction, electron spin-spin contact interaction, electron spin-other-orbit interaction, electron spin-spin dipolar interaction and electron orbit-orbit interaction. The integrals are expressed in suitable forms for use in computer. It is also pointed out that the integrals are written essentially in terms of the overlap, nuclear attraction, electron repulsion, or field integrals.     

Characterisation by rotational spectroscopy of a hydrogen-bonded dimer formed between H2O and HCN

The rotational spectra in the vibrational ground states of (H₂O, HC¹⁴N) and (H₂O, HC¹⁵N) have been assigned in the frequency range 6–19 GHz. Values of rotational constants (B_O, C_O) and centrifugal distortion constants (Δ_J, Δ_JK) have been determined for both species, while the ¹⁴N-nuclear quadrupole coupling constants x_aa and x_bb have been established for the first. Observations concerning additional hyperfine structure arising from H,H nuclear spin-nuclear spin coupling in the H₂O subunit suggest that (H₂O,HCN) has a pair of equivalent protons and is effectively planar in the zero-point state. Observed spectroscopic constants are consistent only with the arrangement H₂O…HCN, with r(O…C) = 3.1387 Å.     

Complexation of copper(II)-Chelidamate: A multifrequency-pulsed electron paramagnetic resonance and electron nuclear double resonance analysis

Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) techniques were used to obtain structural information about the copper(II)-chelidamate complex. Well-resolved nitrogen ENDOR spectra could be recorded from solid solution samples by using selective excitation of spin packets. Evaluation of nuclear quadrupole and dipolar hyperfine interaction of the directly ligated nitrogen allowed for an identification of the bond direction to the copper ion within the eigen frame of the copper g-matrix. Invoking two-dimensional EPR techniques, additional hyperfine interaction with a "distant" nitrogen spin, identified as resulting from the solvent dimethylformamide (DMF), was observed. The experimental data are only consistent with formation of a stable pseudoplanar copper complex with single solvent ligation via its oxygen atom.     

Effect of electronic interactions on NMR 1J(CF) and 2J(CF) couplings in cis- and trans-4-t-butyl-2-fluorocyclohexanones and their alcohol derivatives

In order to study the influence of hyperconjugative, inductive, steric, and hydrogen-bond interactions on (1)J(CF) and (2)J(CF) NMR spin-spin coupling constants (SSCCs), they were measured in cis- and trans-4-t-butyl-2-fluorocyclohexanones and their alcohol derivatives. The four isotropic terms of those SSCCs, Fermi contact (FC), spin dipolar (SD), paramagnetic spin-orbit (PSO), and diamagnetic spin-orbit (DSO), were calculated at the SOPPA(CCSD)/EPR-III level. Significant changes in FC and PSO terms along that series of compounds were rationalized in terms of their transmission mechanisms by employing a qualitative analysis of their expressions in terms of the polarization propagator formalism. The PSO term is found to be sensitive to proximate interactions like steric compression and hydrogen bonding; we describe how it could be used to gauge such interactions. The FC term of (2)J(CF) SSCC in cis-4-t-butyl-2-fluorocyclohexanone is rationalized as transmitted in part by the superposition of the F and O electronic clouds.     

An investigation of the molecular geometry and electronic structure of nitryl chloride by a combination of rotational spectroscopy and ab initio calculations

The ground-state rotational spectra of the six isotopomers (16)O(2) (14)N(35)Cl, (16)O(2) (14)N(37)Cl, (18)O(16)O(14)N(35)Cl, (18)O(2) (14)N(35)Cl, (16)O(2) (15)N(35)Cl, and (16)O(2) (15)N(37)Cl of nitryl chloride were observed with a pulsed-jet, Fourier-transform microwave spectrometer to give rotational constants, Cl and (14)N nuclear quadrupole coupling, and spin-rotation coupling constants. These spectroscopic constants were interpreted to give r(0), r(s), and r(m) ((2)) versions of the molecular geometry and information about the electronic redistribution at N when nitryl chloride is formed from NO(2) and a Cl atom. The r(m) ((2)) geometry has r(N-Cl)=1.8405(6) A, r(N-O)=1.1929(2) A, and the angle ONO=131.42(4) degrees , while the corresponding quantities for the r(s) geometry are 1.8489 A, 1.1940 A, and 131.73 degrees , respectively. Electronic structure calculations at CCSD(T)cc-pVXZ (X=T, Q, or 5) levels of theory were carried out to give a r(e) geometry, vibration-rotation corrections to equilibrium rotational constants, and values of the (35)Cl and (14)N nuclear hyperfine (quadrupole and spin-rotation) coupling constants in good agreement with experiment. The equilibrium geometry at the CCSD(T)/cc-pV5Z level of theory has r(N-Cl)=1.8441 A, r(N-O)=1.1925 A and the angle ONO=131.80 degrees . The observed rotational constants were corrected for the vibration-rotation effects calculated ab initio to yield semiempirical equilibrium constants which were then fitted to give the following semiempirical equilibrium geometry: r(N-Cl)=1.8467(2) A, r(N-O)=1.1916(1) A, and the angle ONO=131.78(3) degrees .     

Electron traps on oxide surfaces: (H+)(e-) pairs stabilized on the surface of 17O enriched CaO.

(H+)(e-) pairs generated at the surface of polycrystalline CaO are analyzed for the first time in terms of the interaction of the unpaired electron spin with the nuclear spin of the 17O anions of the surface. CaO crystals enriched in the 17O isotope are prepared and the corresponding hyperfine coupling constants are measured in electron paramagentic resonance (EPR) spectra. The results are analyzed on the basis of cluster model density functional theory calculations. The computed hyperfine coupling constants for (H+)(e-) pairs formed on the edge, corner, and reverse corner sites of the CaO surface allow a tentative assignment of two observed spectral features to specific morphological surface sites.     

Structure and properties of the (HCl)2H2O cluster observed by chirped-pulse Fourier transform microwave spectroscopy

The rotational spectrum of the cyclic (HCl)(2)H(2)O cluster has been identified for the first time in the chirped pulse, Fourier transform microwave spectrum of a supersonically expanded HCl/H(2)O/Ar mixture. The spectrum was measured at frequencies 6-18.5 GHz, and transitions in two inversion-tunneling states, at close to 1?:?3 relative intensity, have been assigned for the parent species. The two single (37)Cl isotopic species, and the double (37)Cl species have been assigned in the natural abundance sample, and the (18)O and HDO species of the cluster were identified in isotopically enriched samples. The rich nuclear quadrupole hyperfine structure due to the presence of two chlorine nuclei has been satisfactorily fitted and provided useful information on the nonlinearity of intermolecular bonds in the cluster. The r(s) heavy atom geometry of the cluster was determined and the strongest bond in the intermolecular cycle r(O···HCl) = 3.126(3) ?, is found to be intermediate in length between the values in H(2)O···HCl and (H(2)O)(2)HCl. The fitted spectroscopic constants and derived molecular properties are compared with ab initio predictions, and a discussion of complexation effects in these three clusters is made.     

Nuclear spin conversion of methane in solid parahydrogen

The nuclear spin conversion of CH(4) and CD(4) isolated in solid parahydrogen was investigated by high resolution Fourier transform infrared spectroscopy. From the analysis of the temporal changes of rovibrational absorption spectra, the nuclear spin conversion rates associated with the rotational relaxation from the J=1 state to the J=0 state for both species were determined at temperatures between 1 and 6 K. The conversion rate of CD(4) was found to be 2-100 times faster than that of CH(4) in this temperature range. The faster conversion in CD(4) is attributed to the quadrupole interaction of D atoms in CD(4), while the conversion in CH(4) takes place mainly through the nuclear spin-nuclear spin interaction. The conversion rates depend on crystal temperature strongly above 3.5 K for CH(4) and above 2 K for CD(4), while the rates were almost constant below these temperatures. The temperature dependence indicates that the one-phonon process is dominant at low temperatures, while two-phonon processes become important at higher temperatures as a cause of the nuclear spin conversion.     

The study for the incipient solvation process of NaCl in water: the observation of the NaCl-(H2O)n (n = 1, 2, and 3) complexes using Fourier-transform microwave spectroscopy

Pure rotational spectra of the sodium chloride-water complexes, NaCl-(H(2)O)(n) (n = 1, 2, and 3), in the vibronic ground state have been observed by a Fourier- transform microwave spectrometer coupled with a laser ablation source. The (37)Cl-isotopic species and a few deuterated species have also been observed. From the analyses of the spectra, the rotational constants, the centrifugal distortion constants, and the nuclear quadrupole coupling constants of the Na and Cl nuclei were determined precisely for all the species. The molecular structures of NaCl-(H(2)O)(n) were determined using the rotational constants and the molecular symmetry. The charge distributions around Na and Cl nuclei in NaCl are dramatically changed by the complex formation with H(2)O. Prominent dependences of the bond lengths r(Na-Cl) on the number of H(2)O were also observed. By a comparison with results of theoretical studies, it is shown that the structure of NaCl-(H(2)O)(3) is approaching to that of the contact ion-pair, which is considered to be an intermediate species in the incipient solvation process.     

Hydrogen bond geometries from electron paramagnetic resonance and electron-nuclear double resonance parameters: density functional study of quinone radical anion-solvent interactions

Density functional theory was used to study the impact of hydrogen bonding on the p-benzosemiquinone radical anion BQ(*-) in coordination with water or alcohol molecules. After complete geometry optimizations, (1)H, (13)C, and (17)O hyperfine as well as (2)H nuclear quadrupole coupling constants and the g-tensor were computed. The suitability of different model systems with one, two, four, and 20 water molecules was tested; best agreement between theory and experiment could be obtained for the largest model system. Q-band pulse (2)H electron-nuclear double resonance (ENDOR) experiments were performed on BQ(*-) in D(2)O. They compare very well with the spectra simulated by use of the theoretical values from density functional theory. For BQ(*-) in coordination with four water or alcohol molecules, rather similar hydrogen-bond lengths between 1.75 and 1.78 A were calculated. Thus, the computed electron paramagnetic resonance (EPR) parameters are hardly distinguishable for the different solvents, in agreement with experimental findings. Furthermore, the distance dependence of the EPR parameters on the hydrogen-bond length was studied. The nuclear quadrupole and the dipolar hyperfine coupling constants of the bridging hydrogens show the expected dependencies on the H-bond length R(O.H). A correlation was obtained for the g-tensor. It is shown that the point-dipole model is suitable for the estimation of hydrogen-bond lengths from anisotropic hyperfine coupling constants of the bridging (1)H nuclei for H-bond lengths larger than approximately 1.7 A. Furthermore, the estimation of H-bond lengths from (2)H nuclear quadrupole coupling constants of bridging deuterium nuclei by empirical relations is discussed.     

The NaK 1(b) 3Pi(Omega=0) state hyperfine structure and the 1(b) 3Pi(Omega=0) approximately 2(A) 1Sigma+ spin-orbit interaction

We have measured the hyperfine structure of mutually perturbing rovibrational levels of the 1(b) 3Pi0 and 2(A) 1Sigma+ states of the NaK molecule, using the perturbation-facilitated optical-optical double resonance method with copropagating lasers. The unperturbed 1(b) 3Pi0 levels are split into four hyperfine components by the Fermi contact interaction bFIS. Mixing between the 1(b) 3Pi0 and 2(A) 1Sigma+ levels imparts hyperfine structure to the nominally singlet component of the perturbed levels and reduces the hyperfine splitting of the nominally triplet component. Theoretical analysis relates these observations to the hyperfine splitting that each 1(b) 3Pi0 level would have if it were not perturbed by a 2(A) 1Sigma+ level. Using this analysis, we demonstrate that significant hyperfine splitting arises because the 1(b) 3Pi0 state cannot be described as pure Hund's case (a). We determine bF for the 1(b) 3Pi0 levels and also a more accurate value for the magnitude of the singlet-triplet spin-orbit coupling HSO=[1(b) 3Pi0(vb,J)(H(SO))2(A) 1Sigma+(vA,J). Using the known spectroscopic constants of the 1(b) 3Pi state, we obtain bF=0.009 89+/-0.000 27 cm(-1). The values of (H(SO)) are found to be between 2 and 3 cm(-1), depending on vb, vA, and J. Dividing (H(SO)) by calculated vibrational overlap integrals, and taking account of the 1(b) 3Pi(Omega) rotational mixing, we can determine the magnitude of the electronic part H(el) of H(SO). Our results yield (H(el))=(16.33+/-0.15) cm(-1), consistent with our previous determinations using different techniques.     

Microwave spectra and gas phase structural parameters for N-hydroxypyridine-2(1H)-thione

The microwave spectrum for N-hydroxypyridine-2(1H)-thione (pyrithione) was measured in the frequency range 6-18 GHz, providing accurate rotational constants and nitrogen quadrupole coupling strengths for three isotopologues, C(5)H(4)(32)S(14)NOH, C(5)H(4)(32)S(14)NOD, and C(5)H(4)(34)S(14)NOH. Pyrithione was found to be in a higher concentration in the gas phase than the other tautomer, 2-mercaptopyridine-N-oxide (MPO). Microwave spectroscopy is best suited to determine which structure predominates in the gas phase. The measured rotational constants were used to accurately determine the coordinates of the substituted atoms and provided sufficient data to determine some of the important structural parameters for pyrithione, the only tautomer observed in the present work. The spectra were obtained using a pulsed-beam Fourier transform microwave spectrometer, with sufficient resolution to allow accurate measurements of the (14)N nuclear quadrupole hyperfine interactions. Ab initio calculations provided structural parameters and quadrupole coupling strengths that are in very good agreement with measured values. The experimental rotational constants for the parent compound are A = 3212.10(1), B = 1609.328(7), and C = 1072.208(6) MHz, yielding the inertial defect Δ(0) = -0.023 amu·?(2) for the C(5)H(4)(32)S(14)NOH isotopologue. The observed near zero inertial defect clearly indicates a planar structure. The least-squares fit structural analysis yielded the experimental bond lengths R(O-H) = 0.93(2) ?, R(C-S) = 1.66(2) ?, and angle (N-O-H) = 105(4)° for the ground state structure.     

Pulsed EPR investigations of systems modeling molybdenum enzymes: hyperfine and quadrupole parameters of oxo-17O in [Mo 17O(SPh)4]-

Ka band ESEEM spectroscopy was used to determine the hyperfine (hfi) and nuclear quadrupole (nqi) interaction parameters for the oxo-17O ligand in [Mo 17O(SPh)4]-, a spectroscopic model of the oxo-Mo(V) centers of enzymes. The isotropic hfi constant of 6.5 MHz found for the oxo-17O is much smaller than the values of approximately 20-40 MHz typical for the 17O nucleus of an equatorial OH(2) ligand in molybdenum enzymes. The 17O nqi parameter (e2qQ/h = 1.45 MHz, eta approximately = 0) is the first to be obtained for an oxo group in a metal complex. The parameters of the oxo-17O ligand, as well as other magnetic resonance parameters of [Mo 17O(SPh)4]- predicted by quasi-relativistic DFT calculations, were in good agreement with those obtained in experiment. From the electronic structure of the complex revealed by DFT, it follows that the SOMO is almost entirely molybdenum d(xy) and sulfur p, while the spin density on the oxo-17O is negative, determined by spin polarization mechanisms. The results of this work will enable direct experimental identification of the oxo ligand in a variety of chemical and biological systems.     

Single crystal ESR and ENDOR of bis-(salicylaldoximato) Cu(II): Bis-(Salicylaldoximato)Ni(II): Copper,nitrogen and proton hyperfine data and structure of the internal H-bond

An investigation of the ESR and ENDOR spectra of the complex Cusal₂ substituted into single crystals of Nisal₂ is reported. An analysis of the spectra is presented yielding g tensor, the hyperfine interaction tensors of ⁶³Cu, ¹⁴N and four protons and the quadrupole tensor of ¹⁴N, as well as the orientation of the principal axes systems of these quantities. Interpretation of the hyperfine pattern of the set of two perfectly equivalent ¹⁴N nuclei is shown to require a detailed analysis of the second order perturbation terms expressed in the coupled spin basis. For certain orientations of the crystal the ¹⁴N hyperfine pattern shows up to sixteen lines. Analysis of this phenomenon is achieved by exact diagonalization of the energy matrix and is traced back to noncrossing of energy levels. From the magnetic data bonding parameters of Cusal₂ in an extended Maki—McGarvey picture and spin densities on Cu and ¹⁴N are derived. Cusal₂ features an intramolecular hydrogen bridge of type OH…O. Based on some assumptions concerning spin densities, the experimentally determined magnetic constants and a seven-center dipole—dipole interaction model, the structure of the H-bond is inferred to be bent and nonsymmetric, with the angle ∠ OH…O lying in the range 130–143°.