Intermolecular spin–spin coupling constants between P atoms

This paper reports the study by NMR spectroscopy and ab initio methods of the structure of 3,4-dimethyl-1-cyanophosphole and its dimer. The dimer presents a P···P interaction of the pnictogen type due to the presence of σ-holes. NMR of the monomer was recorded in CDCl₃ solution while NMR of the dimer corresponds to the solid state (CPMAS) experiments. The ^2pJ_PP spin–spin coupling constant has not been measured, but calculated at the B3LYP level. AIM, NBO and ELF methodologies have been used to describe the electronic structure of the dimer.     

Ab initio study of the nuclear spin—spin coupling constants and magnetic shielding constants in silicon derivatives via the equations-of-motion method

Non-empirical equations-of-motion calculations of the nuclear spin—spin coupling constants and magnetic shielding constants in a representative series of molecules featuring siliconsilicon or siliconcarbon single, double and triple bonds are presented. The EOM results, which include the main portion of the electron correlation effects, are in resonable agreement with the available experimental data. On passing from single to double and triple bonding situation the pattern for the ¹J(SiY) parameters resembles that exhibited by ¹J(CY) in the structurally related carbocompounds, whereas an inversion in the relative position of the triply bonded atom is predicted in the case of the ²⁹Si resonance relative to the ¹³C sequence.     

Fluorine spin–spin coupling constants of pentafluorobenzene revisited at the ab initio correlated levels

All possible spin–spin coupling constants, ¹⁹F–¹⁹F, ¹⁹F–¹³C, and ¹⁹F–¹H, of pentafluorobenzene were calculated at five different levels of theory, HF, DFT, SOPPA (CCSD), CCSD, and the SOPPA (CCSD)-based composite scheme with taking into account solvent, vibrational, relativistic, and correlation corrections. Most corrections were next to negligible for the long-range couplings but quite essential for the one-bond carbon–fluorine coupling constants. Hartree–Fock calculations were found to be entirely unreliable, while DFT results were comparable in accuracy with the data obtained using the wave function-based methods.     

The optimum contraction of basis sets for calculating spin–spin coupling constants

The previously proposed pcJ-n basis sets, optimized for calculating indirect nuclear spin–spin coupling constants using density functional methods, are re-evaluated for finding the optimum contraction scheme as a compromise between computational efficiency and minimizing contraction errors. An exhaustive search is performed for the H₂, F₂ and P₂ molecules, and candidates for optimum contraction schemes are evaluated for a larger test set of 21 molecules. Using the criterion that the contraction error should not exceed the basis set error relative to the basis set limit, the optimum contraction is defined for each basis set. The results show that it is difficult to contract basis sets for calculating spin–spin coupling constants to any significant degree without losing the inherent accuracy. The work provides guidelines for searching for optimum contraction schemes for other properties and/or at theoretical levels where a systematic search is impractical.     

Calculations of 13C NMR chemical shifts and F–C coupling constants of ciprofloxacin

Ciprofloxacin is a widely used fluoroquinolone antibiotic. In this work, a comprehensive evaluation of MP2 and DFT with different functionals and basis sets was carried out to select the most suitable level of theory for the study of the NMR properties of ciprofloxacin. Their relative predictive capabilities were evaluated comparing the theoretically predicted and experimental spectral data. Our computational results indicated that in contrast to the solid state, the molecule of ciprofloxacin does not exist as a zwitterion in gaseous state. The results of the calculations of the chemical shifts most close to the experimental were obtained with B3LYP/aug-cc-pVDZ. The F–C coupling constants were calculated systematically with different DFT methods and several basis sets. In general, the calculations of the coupling constants with the BHandH computational method including the applied in this work 6-311++G**, EPRII, and EPRIII basis sets showed a good reproducibility of the experimental values of the coupling constants.     

Correlation of the 13C13C spin—spin coupling constants with the stretching force constants of single and double carbon—carbon bonds

Linear correlations between the spin—spin carbon—carbon coupling constants and the carbon—carbon stretching force constants for single and double bonds have been found through analysis of the available literature data. The corresponding equations are K^s_CC = 0.0344 J^s_CC+3.25 and K^d_CC = 0.180 J^d_CC−3.25 for single and double bonds respectively.     

Vibronic spin—orbit coupling in the phosphorescence of 4-hydroxypyridine

A phosphorescence emission and polarization study of 4-hydroxypyridine has been performed in EPA and in alkaline EPA. In contrast to the lack of luminescence from pyridine, 4-hydroxypyridine phosphoresces with φ_p = 0.40 (77°K) in EPA and τ_p = 0.27 sec. The 00 band of phosphorescence in EPA is clearly resolved at 29,940 cm⁻¹ and is out of plane polarized. An out of plane vibronic perturbation is evident in the polarization curve and is believed to arise from second order effects involving vibronic spin—orbit coupling. The out of plane vibration contributes in plane polarization outside the 00 band. The positive polarization of the 00 band in alkaline EPA is attributed to the inversion of two close lying perpendicularly polarized singlet states. No phosphorescence was observed in methylcyclohexane. The lowest triplet is assigned as π,π*³.     

NMR chemical shielding and spin–spin coupling constants across hydrogen bonds in uracil–α-hydroxy-N-nitrosamine complexes

One- and two bond spin–spin coupling constants, ¹ J, ^1h J , and ^2h J across X–H?O hydrogen bonds and shielding constants of bridging hydrogens have been computed for complexes formed from interaction between the α-hydroxy-N-nitrosamine (NP) and four preferential binding sites of the uracil (U) at B3LYP/6-311++G(2d,2p)//MP2/6-311++G(2d,2p) level of theory. All complexes are stabilized by two H_U?O_NP and H_NP?O_U hydrogen bonds. Very good correlations were found between NMR spin–spin coupling constant as well as isotropic shielding constant and the binding energy, H-bond distance, red-shift of vibration frequency, charge transfer energy, and electron density at H-bond critical point.     

Hydrogen bonding of the hydroxyl group and spin—spin coupling of the ring protons of substituted phenols

The chemical shifts and coupling constants of the hydroxyl and ring protons of the following substituted phenols and anisoles have been obtained in basic and nonbasic solvents: 2-adamant-1-yl-4-tert-butylphenol, 6-bromo-2-adamant-1-yl-4-tert-butylphenol, 3,5-di-tert-butylphenol, 3,5-dichlorophenol, 6-bromo-2-adamant-1-yl-4-tert-butyl anisole and 6-bromo-2,4-di-tert-butyl anisole. Hydrogen bonding of the hydroxyl group of the substituted phenols to the solvent does not cause any detectable change in the spin—spin coupling between the meta protons, but causes a very small increase (<0.10 Hz) in the coupling between the ortho and para protons.     

31P and 17O nuclear spin—lattice relaxation in some phosphorylated molecules. Determination of 31P spin—rotation coupon constants, 17O quadrupolar coupling constants and chemical shielding anisotropy of the 31P nucleus

The spin—lattice relaxation time of the ³¹P nucleus was measured for 11 phosphorylated molecules (phosphine oxides, trialkylphosphates and phosphoramides) dissolved in nitromethane at three different frequencies and as a function of the temperature for three compounds. The different contributions to the relaxation rate due to dipolar, chemical shift anisotropy and spin—rotation interactions were determined and the reorientational correlation times of the molecules were deduced when the anisotropy of the chemical shift tensor of the ³¹P nucleus could be (re)determined. The quadrupolar coupling constant of the ¹⁷O nucleus was also determined from the linewidth of the nuclear magnetic resonance signals, for phosphine oxides and triphenylphosphate, giving some information on the electronic distribution into the phosphoryl bond. The spin—rotation coupling constants for trimethylphosphine oxide and triphenylphosphine oxide were deduced and the chemical shift anisotropy Δσ of trialkylphosphates estimated.     

The linear relationship between the one-bond spin—spin coupling constants 1JCC and the product of the electronegativities Ex·Ey of substituents at the CC triple bond

The linear equation connecting the one-bond spin–spin coupling constants between carbon nuclei, J_CC, and the product of the electronegativities of substituents at the CC triple bond is derived using a large number of data for variously substituted acetylenes. The equation ¹J_CC = 23.23 E_x·E_y + 15.45 provides a means for estimation of unknown ¹J_CCs and for calculation (and/or verification) of electronegativities of substituents. The equation allows one to estimate a total range of about 350 Hz for ¹J_CCs. The smallest value (30 Hz) is predicted for dicaesium acetylide, Cs₂C₂, and the largest one for difluoroacetylene, F₂C₂ (383 Hz). The electronegativities of tin and lead, which constituted a subject of long-lasting controversy in the literature, calculated using the equation are equal to 1.74 and 1.64, respectively.     

One-bond 1J(15N─19F) spin–spin coupling constants of cationic fluorinating reagents: Insights from DFT calculations

We have investigated, by means of density functional theory protocols, the one-bond ¹J(¹⁵N─¹⁹F) spin–spin coupling constants in a series of fluorinating reagents, containing the N─F bond, recently studied experimentally. The results of the calculations show a very good linear relationship with the experimental values, even though only the M06-2X(PCM)/pcJ-2//B3LYP/6-311G(d,p) level affords a very low mean absolute error. The calculations allow to analyze the various molecular orbitals contributions to the J coupling and to rationalize the observed positive sign, corresponding to a negative sign of the reduced spin–pin coupling constant K(N─F). Moreover, of the four Ramsey contributions, only the diamagnetic spin orbit is negligible, whereas the paramagnetic spin orbit and spin dipole terms decrease the magnitude of the Fermi contact (FC) term by an amount that goes from a minimum of 35% up to more than 60% of the FC term itself. Several effects have been investigated, namely, the contribution of the long-range solvent reaction field, relativistic corrections, and conformational and vibrational effects.     

Influence of stereoelectronic effects on the 1JC─F spin–spin coupling constant in fluorinated heterocyclic compounds

The Perlin effect and its analog for fluorinated compounds (the fluorine Perlin-like effect) manifest on one-bond C─H (C─F for the fluorine Perlin-like effect) spin–spin coupling constants (SSCCs) in six-membered rings. These effects can be useful to probe the stereochemistry (axial or equatorial) of the C─H and C─F bonds, respectively. The origin of these effects has been debatable in the literature as being due to hyperconjugative interactions, dipolar effects, and induced current density. Accordingly, a variety of model compounds has been used to probe such effects since the cyclohexanone carbonyl group and the endocyclic heteroatom lone pairs play different roles on the above-mentioned effects. Thus, the ¹J_C─F SSCC in fluorinated lactams and lactones were theoretically studied to gain further insight on the nature of the fluorine Perlin-like effect. In addition, because the intramolecular α-effect has recently gained attention for its importance in the reactivity and stereoelectronic interactions in peroxide compounds, some fluorinated 1,2-dioxanes and 1,2-dithianes were studied to evaluate the role of the α-effect on the behavior of ¹J_C─F SSCCs. Differently from fluorinated ketones and ethers, the fluorine Perlin-like effect in the amides and esters cannot be explained by hyperconjugative or dipolar interactions alone, because the resonance in these groups affect the ¹J_C─F values. The O─O and S─S-containing systems exhibit a strong fluorine Perlin-like effect, but unlike the α-effect, this behavior cannot be explained neither by hyperconjugation nor by dipolar interactions alone; the spatial proximity of the C─F and O─O/S─S bonds is proposed to affect the magnitude of the ¹J_C─F SSCC.     

One-bond CC spin—spin coupling constants in derivatives of benzene. Non-linearity of 1J(CC) vs substituent electronegativity

A set of one-bond CC coupling constants has been determined for mono- and disubstituted benzenes. Large ¹J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive substituents. The total range of ¹J(CC) couplings observed in our work is larger than 50 Hz. These large variations of CC spin—spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between ¹J(CC) and the substituent electronegativity is not linear.     

Tetra-hydrides of the third-row transition elements: spin–orbit coupling effects on geometrical deformation in WH4 and OsH4

The dissociation energies of MH₄ (M =  La, Hf–Hg) were computed using full optimized reaction space (FORS) multi-configuration self-consistent field (MCSCF) and second-order multi-reference Møller–Plesset perturbation methods with the SBKJC basis sets augmented by a set of polarization functions (SBKJC(f,p)). It was shown that of the molecules examined, only four tetra-hydrides HfH₄, TaH₄, WH₄, and OsH₄ with T_d symmetry are lower in energy than the corresponding dissociation limits. For WH₄ and OsH₄, the potential energy surfaces from the D_4h to the T_d structure were explored from both theoretical calculations and symmetry arguments based on the pseudo-Jahn- Teller effect. As for WH₄, it is found that the ground state could be ³E_g, ³A_2g, or ³B_2g at the D_4h structure. The present calculations suggest that the ground state is ³E_g, and that this state is stabilized by the e_u deformation into a C_2v structure (³B₁) and then sequentially to the most stable T_d structure (³A₂). If the molecular system is promoted to the lowest ³B_2g state, the D_4h structure can directly deform into the most stable T_d structure along the b_2u vibrational mode. For OsH₄, the ground state (⁵B_1g) at the D_4h structure deforms into a D_2d structure and the resulting ⁵B₂ state strongly interacts with the lowest ³E and ¹A₁ states due to the spin-orbit couplings (SOCs). As a result, it was shown that the relativistic potential energy of the lowest spin-mixed state (ground state) monotonically decreases along the D_2d deformation path from the D_4h to the T_d structure.     

Ab initio calculations for the potential curves and spin–orbit coupling of Mg2

 The ground state and several low-lying excited states of the Mg₂ dimer have been studied by means of a combination of the complete-active-space multiconfiguration self-consistent-field (CASSCF)/CAS multireference second-order perturbation theory (CASPT2) method and coupled-cluster with single and double excitations and perturbative contribution of connected triple excitations [CCSD(T)] scheme. Reasonably good agreement with experiment has been obtained for the CCSD(T) ground-state potential curve but the dissociation energy of the only experimentally known A¹Σ _u ⁺ excited state of Mg₂ is somewhat overestimated at the CASSCF/CASPT2 level. The spectroscopic constants D _e, R _e and ω_e deduced from the calculated potential curves for other states are also reported. In addition, some spin–orbit matrix elements between the excited singlet and triplet states of Mg₂ have been evaluated as a function of internuclear separation. Received: 10 May 2001 / Accepted: 15 August 2001 / Published online: 30 October 2001     

The quantum chemical calculation of NMR two-bond spin–spin coupling constants in the N?···H–OH?→?N–H···OH? switching

A quantum chemical study has been carried out to investigate the effects of the size of H₂O cluster and substituents (X = H, Me, OMe, CHO, NO, and NO₂) in the para position of the anilide ion on the two-bond spin–spin coupling constants (SSCCs) ^2h J _N···O across in the N–H–O switching at B3LYP/6-311 ++G(2d,2p) level of theory. The changes in ^2h J _N···O SSCCs due to variation of substituent and H₂O cluster size were well monitored by changes in binding energy, structural parameter, electron density topography, natural charge, charge transfer, and percentage of p-character of N atom in the C–N bond. Linear correlations were found between ^2h J _N···O and above-mentioned properties.     

Stereochemical study of the sterically crowded phenylselanylalkenes by means of 77Se?1H spin–spin coupling constants

Stereochemical study of five sterically crowded phenylselanylalkenes obtained via the hydroselenation of either terminal or internal alkynes with benzeneselenol catalyzed by the nanosized Ni complexes has been carried out based on the experimental HMBC measurements and theoretical second order palarization propagator approach (SOPPA) calculations of their ⁷⁷Se? ¹H spin–spin coupling constants across double bond in combination with the energy‐based theoretical conformational analysis performed at the MP2/6‐311G** level. It has been found that studied phenylselanylalkenes adopt mainly skewed s‐cis conformation with the noticeable out‐of‐plane deviations of the phenylselanyl and phenyl groups. Copyright © 2011 John Wiley & Sons, Ltd.     

The effect of molecular distortions on spin–orbit coupling in simple hydrocarbons

Recent work [D.N.S. Parker et al., Chem. Phys. Lett. 469 (2009) 43–49] has found intersystem crossing (ISC) on an ultrafast timescale in electronically excited benzene, a surprise as hydrocarbons generally have small spin–orbit coupling. In this paper, the effect of molecular distortions on spin–orbit coupling (SOC) is calculated for cyclobutadiene and benzene. At equilibrium the SOC in both molecules is negligible, and therefore terms arising from molecular distortions must play a significant role in any fast ISC. We show that out-of-plane C–H bends, which leads to the hybridisation of σ and π orbitals, are responsible for the most significant effect. The S₁/S₀ conical intersection is an important feature for understanding the photochemistry of these molecules. We examine the SOC along the vector from the Franck–Condon point to the lowest energy point on the crossing seam and discuss the potential importance of the SOC to the ultrafast dynamics.