NH Bond Lengths Research Articles

Synthesis, molecular structure, conformational, and intramolecular hydrogen bond strength of ethyl 3-amino-2-butenoate and its N-Me, N-Ph, and N-Bn analogs; An experimental and theoretical study

Amino derivatives of ethyl acetoacetate like ethyl (Z)- 3-(amino)but-2-enoate (EAB), ethyl (Z)- 3-(methylamino)but-2-enoate (Me-EAB), ethyl (Z)- 3-(phenylamino)but-2-enoate (Ph-EAB), and ethyl (Z)- 3-(benzylamino)but-2-enoate (Bn-EAB) were synthesized. Their structures, conformations, and intramolecular hydrogen bonding (IHB) is characterized using computational analysis (density functional theory, DFT, methods) at the B3LYP/6-311++G(d,p) level and spectroscopic techniques)IR, UV, and NMR). All the mentioned theoretical and experimental results were compared to those of (Z)-methyl 3-aminobut-2-enoate (MAB). The vibrational spectra of EAB are fully assigned. Vibrational spectroscopy results confirmed the existence of two conformers of title molecules. DFT calculations at the B3LYP/6-311++G(d,p) level show the calculated NH bond lengths in EAB, Me-EAB, Ph-EAB, Bn-EAB, and MAB are 1.015, 1.017, 1.022, 1.018, and 1.014 Ǻ, respectively. Also, according to the natural bond orbital (NBO) results, the σN-H10↔LP(2)O7 interactions for EAB, Me-EAB, Ph-EAB, Bn-EAB, and MAB are 5.21, 6.77, 8.65, 7.18, and 5.11 kcal/mol, respectively. So, the trend in IHB strength is: Ph-EAB > Bn-EAB ∼ Me-EAB > EAB ∼ MAB, which also agrees with the mentioned experimental results. The mentioned trend also is confirmed by the LP(O7)↔σNH steric exchange interaction.

Pyrene functionalized oxacalix[4]arene architecture as dual readout sensor for expeditious recognition of cyanide anion.

A pyrene functionalized oxacalix[4]arene architecture (DPOC) was utilized as a fluorescence probe for selective recognition of cyanide ions. The receptor DPOC shows excellent selectivity towards cyanide ion with a red shift of 108nm in absorption band along with a significant change in colour from light yellow to pink. The fluorescence titration experiments further confirm the lower limit of detection as 1.7µM with no significant influences of competing anions. 1H-NMR titration experiments support the deprotonation phenomena, as the -NH proton disappears upon successive addition of cyanide ions. The DFT calculation also indicates a certain increment of -NH bond length upon interaction with cyanide ions. The spectral properties as well as colour of DPOC-CN- system may be reversed upon the addition of Ag+/ Cu2+ ions up to 5 consecutive cycles. Moreover, DPOC coated "test strips" were prepared for visual detection of cyanide ions.

Catalytic properties of nano-brass clusters: A density functional theory study

Brass (Cu-Zn) alloys are widely used as industrial materials. The structural and electronic properties of Cu-Zn clusters have been reported in theory recently [(Liu and Cheng, 2019); Journal of Cluster Science 2020, 31, 601–607], and the works have attracted intense research interest both experimentally and theoretically. However, to our knowledge the catalytic properties of the Cu-Zn clusters have not been analyzed. Here, we carry out our explorations of the catalytic properties of the selected Cu-Zn superatoms related to their geometric structures and stability. First, the global minimum structures of CuxZny (x + y = 14) alloy clusters are searched by an unbiased global method of genetic algorithm directly using the DFT functional. Among them, the spherical Cu8Zn6 and Cu10Zn4 satisfy magic numbers of the Jellium model and are two stable metal superatoms in this cluster series. Then, electrostatic potential surfaces of the brass clusters are analyzed, and the calculated results show that such alloy superatom has remarkable σ-holes with positive potential regions. Moreover, these electron-deficient regions can be considered as interaction sites with some Lewis bases. When a Lewis base CO gas molecule is attached to the alloy superatoms, we found that the CO bond distance become slightly elongated and its stretching frequency presents a significant red-shift. This is because that delocalized electrons of brass superatoms transfer towards the anti-bond π orbitals of CO molecule. Hence, these clusters may serve as potential catalysts for the covalent bond activation. However, the NH bond lengths of the clusters upon the NH3 adsorption are in accordance with that of the free NH3, and the analysis shows that the classical metals bind the NH3 molecule via donor interactions.

One-bond 1 J(15 N,H) coupling constants at sp2 -hybridized nitrogen of Schiff bases, enaminones and similar compounds: A theoretical study.

1 J(15 N,H) coupling constants for enaminones and NH-forms of intramolecularly hydrogen-bonded Schiff bases as model compounds for sp2 -hybridized nitrogen atoms are evaluated using density functional theory (DFT) to find the optimal functionals and basis sets. Ammonia is used as a test molecule and its one-bond coupling constant is compared with experiment. A methylamine Schiff base of a truncated molecule of gossypol is used for checking the performance of selected B3LYP, O3LYP, PBE, BHandH, and APFD density functionals and standard, modified, and dedicated basis sets for coupling constants. Both in vacuum and in chloroform, modeled by the simple continuum model of solvent, the modified basis sets predict significantly better the 1 J(15 N,H) value in ammonia and in the methylamine Schiff base of a truncated molecule of gossypol than the standard basis sets. This procure is then used on a broad set of intramolecularly hydrogen-bonded molecules, and a good correlation between calculated and experimental one-bond NH coupling constants is obtained. The 1 J(15 N,H) couplings are slightly overestimated. The calculated data show for hydrogen-bonded NH interatomic distances that the calculated values depend on the NH bond lengths. The shorter the bond lengths, the larger the 1 J(15 N,H). A useful correlation between 1 J(15 N,H) and NH bond length is derived that enables realistic predictions of one-bond NH coupling constants. The calculations reproduce experimentally observed trends for the studied molecules.

Fast Acquisition of Proton-Detected HETCOR Solid-State NMR Spectra of Quadrupolar Nuclei and Rapid Measurement of NH Bond Lengths by Frequency Selective HMQC and RESPDOR Pulse Sequences.

Fast magic-angle spinning (MAS), frequency selective (FS) heteronuclear multiple quantum coherence (HMQC) experiments which function in an analogous manner to solution SOFAST HMQC NMR experiments, are demonstrated. Fast MAS enables efficient FS excitation of 1 H solid-state NMR signals. Selective excitation and observation preserves 1 H magnetization, leading to a significant shortening of the optimal inter-scan delay. Dipolar and scalar 1 H{14 N} FS HMQC solid-state NMR experiments routinely provide 4- to 9-fold reductions in experiment times as compared to conventional 1 H{14 N} HMQC solid-state NMR experiments. 1 H{14 N} FS resonance-echo saturation-pulse double-resonance (RESPDOR) allowed dipolar dephasing curves to be obtained in minutes, enabling the rapid determination of NH dipolar coupling constants and internuclear distances. 1 H{14 N} FS RESPDOR was used to assign multicomponent active pharmaceutical ingredients (APIs) as salts or cocrystals. FS HMQC also provided enhanced sensitivity for 1 H{17 O} and 1 H{35 Cl} HMQC experiments on 17 O-labeled Fmoc-alanine and histidine hydrochloride monohydrate, respectively. FS HMQC and FS RESPDOR experiments will provide access to valuable structural constraints from materials that are challenging to study due to unfavorable relaxation times or dilution of the nuclei of interest.

Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state.

Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV-UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA-H2O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S0-S1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm-1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NHO[double bond, length as m-dash]C H-bond, which shifts the 3nπ* state well above the 1ππ* S1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck-Condon activity due to three in-plane vibrations that modulate the distance between the NH2 and CO2Me groups, ν33 (421 cm-1), ν34 (366 cm-1), and ν36 (179 cm-1). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NHO[double bond, length as m-dash]C H-bond. This leads to a shortening of the NHO[double bond, length as m-dash]C H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the HO distance compared to full H-atom transfer. As a result, the excited state process near the S1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value.

Intramolecular hydrogen bonding promoted excited state double proton transfer mechanism based on a typical molecule: Porphycene

The double excited state intramolecular proton transfer (ESIPT) mechanisms of porphycene, were theoretically studied. The primary bond lengths, IR vibrational spectra and hydrogen-bond energy indicate that the intramolecular hydrogen bonds were strengthened in the first excited state, which facilitate the ESIPT processes. To elucidate the proposed mechanism, the potential-energy surfaces of the ground state and first excited state were constructed as functions of NH bond lengths and its relative torsional angle rotation. The intramolecular proton transfer of prophycene is more likely to occur through lengthwise pathway and proceed in the concerted coordinated transfer manner.

Correlation of the bond-length change and vibrational frequency shift in model hydrogen-bonded complexes of pyrrole

An MP2 computational study of model hydrogen-bonded pyrrole⋯YZ (YZ=NH3, NCH, BF, CO, N2, OC, FB) complexes was undertaken in order to examine the variation of the NH bond length change and its associated vibrational frequency shift. The chemical hardness of Y, as well as the YZ dipole moment, were found to be important parameters in modifying the bond length change/frequency shift. The basis set effect on the computed properties was also assessed. A perturbative model, which accurately reproduced the ab initio NH bond length changes and frequency shifts, was useful in rationalizing the observed trends.

Molecular structure, vibrational spectra, NBO analysis and molecular packing prediction of 3-nitroacetanilide by ab initio HF and density functional theory

Quantum chemical calculations of geometries and vibrational wavenumbers of 3-nitroacetanilide (C8H8N2O3) in the ground state were carried out by using ab initio HF and density functional theory (DFT/B3LYP) methods with 6-31+G* basis set. The -311++G** basis set is also used for B3LYP level. The scaled harmonic vibrational frequencies have been compared with experimental FT-IR spectra. Theoretical vibrational spectra of the title compound were interpreted by means of potential energies distributions (PEDs) using MOLVIB program. The theoretical spectrograms for IR spectra of the title compound have been constructed. The shortening of CH bond length and the elongation of NH bond length suggest the existence of weak CH⋯O and NH⋯O hydrogen bonds, which is confirmed by the natural bond orbital analysis. In addition, the crystal structure obtained by molecular mechanics belongs to the P21 space group, with lattice parameters Z=4, a=14.9989Å, b=4.0367Å, c=12.9913Å, ρ=0.998gcm−3.

DFT/B3LYP study of the substituent effects on the reaction enthalpies of the antioxidant mechanisms of Indole-3-Carbinol derivatives in the gas-phase and water

In this paper the reaction enthalpies of three antioxidant action mechanisms, HAT, SET-PT and SPLET, for mono-substituted Indole-3-Carbinols were calculated in gas-phase and water. Results show that electron-withdrawing substituents increase the bond dissociation enthalpy (BDE), ionization potential (IP) and electron transfer enthalpy (ETE), while electron-donating ones cause a rise in the proton dissociation enthalpy (PDE) and proton affinity (PA). In comparison to gas-phase, water attenuates the substituent effect on all reaction enthalpies. Results show that IP and BDE values can be successfully correlated with the indolic NH bond length after electron abstraction, R(NH+), and the partial charge on the indolyl radical nitrogen atom, q(N). Furthermore, calculated IP and PA values for mono-substituted Indole-3-Carbinols show linear dependence on the energy of the highest occupied molecular orbital (EHOMO) of studied molecules in the two environments. SPLET represents the thermodynamically preferred mechanism in water.

Temperature-Dependent, Effective Structures of the 14NH3 and 14ND3 Molecules

Measurements result in effective, usually temperature-dependent structural parameters of molecules, and never directly in equilibrium structures, which are theoretical constructs. A recent high-accuracy semiglobal potential energy surface of the electronic ground state of the ammonia molecule, called NH3-Y2010 (J. Mol. Spectrosc. 2011, 268, 123), which exhibits mass-independent equilibrium NH bond length and a HNH bond angle of 1.0109 Å and 106.75°, respectively, is employed together with the variational nuclear motion code GENIUSH (J. Chem. Phys. 2009, 130, 134112; 2011, 134, 074105) to determine directly measurable, effective structural parameters of the (14)NH(3) and (14)ND(3) molecules. The effective r(g)- and r(a)-type NH(ND) distances determined at 300 K are 1.0307(1.0254) and 1.0256(1.0217) Å, respectively, with an estimated accuracy of 2 × 10(-4) Å. The effective θ(g) HNH and DND bond angles at 300 K are 106.91° and 106.85°, respectively. The root-mean-square amplitudes of vibration, l(g), for the NH(ND) distances at 300 K are 0.073(0.062) Å. These structural parameters confirm the less accurate results of a room-temperature gas-electron-diffraction study (J. Chem. Phys. 1968, 49, 2488, all data in Å): r(g)(NH) = 1.030(2), l(g)(NH) = 0.073(2), r(g)(ND) = 1.027(3), and l(g)(ND) = 0.061(2). The computed difference in the r(g,T)(NH) bond lengths of the two spin isomers (ortho and para forms) of (14)NH(3) is 3 × 10(-5) Å at 0 K, the difference diminishes at temperatures of about 30-50 K.

Deuterium isotope effects on 13C and 15N chemical shifts of intramolecularly hydrogen-bonded enaminocarbonyl derivatives of Meldrum’s and Tetronic acid

Secondary deuterium isotope effects on 13C and 15N nuclear shieldings in a series of cyclic enamino-diesters and enamino-esters and acyclic enaminones and enamino-esters have been examined and analysed using NMR and DFT (B3LYP/6-31G(d,p)) methods. One-dimensional and two-dimensional NMR spectra of enaminocarbonyl and their deuterated analogues were recorded in CDCl 3 and CD 2Cl 2 at variable temperatures and assigned. 1 J NH coupling constants for the derivatives of Meldrum’s and tetronic acids reveal that they exist at the NH-form. It was demonstrated that deuterium isotope effects, for the hydrogen bonded compounds, due to the deuterium substitution at the nitrogen nucleus lead to large one-bond isotope effects at nitrogen, 1Δ 15N(D), and two-bond isotope effects on carbon nuclei, 2ΔC(ND), respectively. A linear correlations exist between 2ΔC(ND) and 1Δ 15N(D) whereas the correlation with δNH is divided into two. A good agreement between the experimentally observed 2ΔC(ND) and calculated dσ 13C/dR NH was obtained. A very good correlation between calculated NH bond lengths and observed NH chemical shifts is found. The observed isotope effects are shown to depend strongly on Resonance Assisted Hydrogen bonding.

Proton transfer in the intramolecular NHN+ bonds in proton sponges with different hydrogen bridge flexibility

The proton transfer in the intramolecular NHN(+) hydrogen bonds of selected proton sponges has been studied using theoretical calculations of the potential energy surfaces (PES). The proton-transfer trajectory follows very closely the lowest energy path, derived as the quantum-mechanical reaction coordinates (QMRC). The bond order is not conserved in the transfer process. Even in the most flexible proton sponges there are considerable constraints on the NN distance and the hydrogen bonds do not behave as intermolecular bonds. The curvature of QMRC is not a suitable criterion to distinguish between inter- and intramolecular NHN(+) bonds. It appears that the determining factor for linearity is the degree of constraint, which is most likely the strongest in the benzene proton sponges. In the naphthalene proton sponges with relatively short NN distances QMRC is more bent than in the benzene complexes with somewhat longer distances, opposite to what might be expected. It is important to note that in intramolecular complexes the PES is characterized by a single minimum, in contrast to a double minimum in intermolecular complexes. The experimentally determined NH bond lengths have been plotted on the potential energy surface and these points are all located on the QMRC curve, very close to the energy minimum of the PES. However, it is vital that the experimental X-ray hydrogen positions are then corrected to give the true internuclear NH distances.

Variability of the 15N Chemical Shielding Tensors in the B3 Domain of Protein G from 15N Relaxation Measurements at Several Fields. Implications for Backbone Order Parameters

We applied a combination of 15N relaxation and CSA/dipolar cross-correlation measurements at five magnetic fields (9.4, 11.7, 14.1, 16.4, and 18.8 T) to determine the 15N chemical shielding tensors for backbone amides in protein G in solution. The data were analyzed using various model-independent approaches and those based on Lipari-Szabo approximation, all of them yielding similar results. The results indicate a range of site-specific values of the anisotropy (CSA) and orientation of the 15N chemical shielding tensor, similar to those in ubiquitin (Fushman, et al. J. Am. Chem. Soc. 1998, 120, 10947; J. Am. Chem. Soc. 1999, 121, 8577). Assuming a Gaussian distribution of the 15N CSA values, the mean anisotropy is -173.9 to -177.2 ppm (for 1.02 A NH bond length) and the site-to-site CSA variability is +/-17.6 to +/-21.4 ppm, depending on the method used. This CSA variability is significantly larger than derived previously for ribonuclease H (Kroenke, et al. J. Am. Chem. Soc. 1999, 121, 10119) or recently, using "meta-analysis" for ubiquitin (Damberg, et al. J. Am. Chem. Soc. 2005, 127, 1995). Standard interpretation of 15N relaxation studies of backbone dynamics in proteins involves an a priori assumption of a uniform 15N CSA. We show that this assumption leads to a significant discrepancy between the order parameters obtained at different fields. Using the site-specific CSAs obtained from our study removes this discrepancy and allows simultaneous fit of relaxation data at all five fields to Lipari-Szabo spectral densities. These findings emphasize the necessity of taking into account the variability of 15N CSA for accurate analysis of protein dynamics from 15N relaxation measurements.

Theoretical Elucidation of the Unusually High [HNC]/[HCN] Abundance Ratio in Interstellar Space: Two‐dimensional and Two‐State Quantum Wave Packet Dynamics Study on the Branching Ratio of the Dissociative Recombination Reaction HCNH++e−→ HNC/HCN + H

In order to elucidate the unusually high [HNC]/[HCN] abundance ratio observed in interstellar space, we have carried out wave packet simulations for the dissociative recombination reaction HCNH+ + e- → HNC/HCN + H on the two-dimensional potential energy surfaces (PESs) of two dissociative electronic states of HCNH, 1 2Σ+ and 2 2Σ+. Two adiabatic PESs of 1 2Σ+ and 2 2Σ+ have been determined as functions of NH and CH bond lengths for a linear geometry of HCNH with a fixed CN bond length, by the multireference single and double configuration interaction plus Davidson's quadruple excitation correction [MR-SDCI(+Q)] method. The wave packets, generated from the low-lying vibrational eigenstates of HCNH+, have been put initially on the 2 2Σ+ adiabatic PES. The resulting branching ratios of [HNC]/[HCN] vary from 0.77 to 1.32, depending on the initial vibrational quantum numbers, which support that HCNH+ is the precursor of both HNC and HCN in interstellar space.

Coordination of 2,3-diaminopyridine in the diamidopyridinium mode to the ReO3+ core

2,3-Diaminopyridine (H2dap) reacts with (n-Bu4N)[ReOCl4] to form an air-stable salt of composition [ReO(Hdap)2]Cl·2H2O. Spectroscopic studies and X-ray crystallography show that Hdap is coordinated in the monoanionic diamidopyridinium mode to the oxorhenium(V) center. The protonation of the pyridyl nitrogens and the charge on the complex have no influence on the bond lengths and angles, with the rhenium(V)–oxo and average Re–NH bond lengths equal to 1.688(4) and 1.993(5) Å, respectively.

Structural Dependence of NH Stretch Mode Frequency Shifts in Amide and Peptide Systems

The NH stretch (s) frequency, ν(NH), in peptides and proteins, as in many amide group hydrogen bond (HB) systems, is used as an indicator of the presence and strength of the bond. This is based on the concept that ν(NH) is associated with an essentially localized mode whose frequency is influenced only by the HB interaction. We examine this assumption through ab initio studies of N-methylacetamide (NMA) and of 78 conformers of the alanine dipeptide (ADP). Based on our recent studies of nitrogen pyramidalization in NMA (J. Phys. Chem. A 2003, 107, 1825), we find that such nonplanarity, whether deriving from a nonplanar peptide group or only from a steric constraint, results in an increase in the NH bond length, r(NH), and thus in a decrease in ν(NH). This is accompanied by changes in other internal coordinates that make small contributions to this mode, emphasizing that although NH s is a highly localized mode it is not completely so. It is not surprising, therefore, to find that changes in the environment of the NH group, such as exist in the ADP conformers, lead to variations in ν(NH) even in the absence of traditional HB configurations. The ADP results permit an analysis of these effects and also illustrate the important role, as we recently emphasized (J. Phys. Chem. A 2002, 106, 11663), of electrical interactions in affecting r(NH) and thus ν(NH). We show how the increased strength of this interaction in ADP conformers with “standard” HBs leads, through the much larger induced dipole derivative, to the large increase in infrared intensity of NH s bands in such structures.

On the mechanism of nonradiative decay of DNA bases: ab initio and TDDFT results for the excited states of 9H-adenine

Minimum-energy reaction paths and corresponding potential-energy proles have been com- puted for the lowest excited states of the amino form of 9H-adenine. Complete-active- space self-consistent- eld (CASSCF) and density functional theory (DFT) methods have been employed. The potential-energy function of the lowest 1 state, nominally a 3s Rydberg state, is found to be dissociative with respect to the stretching of the NH bond length of the azine group. The 1 potential-energy function inter- sects not only those of the 1 and 1 n excited states, but also that of the electronic ground state. The 1 { 1 and 1 {S0 intersections are converted into conical intersections when the out-of-plane motion of the active hydrogen atom is taken into account. It is argued that the predissociation of the 1 and 1 n states by the 1 state and the conical intersection of the 1 state with the S0 state provide the mechanism for the ultrafast radiationless deactivation of the excited singlet states of adenine.

(15)N-(1)H bond length determination in natural abundance by inverse detection in fast-MAS solid-state NMR spectroscopy.

A solid-state 15N-1H correlation NMR experiment is presented, which provides a substantial gain in signal sensitivity by 1H inverse detection under fast MAS conditions and allows for the precise determination of NH bond lengths via heteronuclear 1H-15N dipole-dipole couplings on samples naturally abundant in 15N. Pulsed-field gradients or, alternatively, radio frequency pulses ensure suppression of unwanted 1H signals. In this way, natural-abundance 15N-1H correlation NMR spectroscopy becomes feasible in the solid state with experiment times of a few hours. The dipole-dipole coupling constants are extracted from spinning sideband patterns generated by recently developed recoupling strategies. The information on 15N/1H chemical shifts and quantitative 15N-1H couplings can readily be combined in a single two-dimensional spectrum using a split-t1 approach.

To What Extent Do External Fields and Vibrational and Isotopic Effects Influence NMR Coupling Constants Across Hydrogen Bonds? Two-Bond Cl−N Spin−Spin Coupling Constants (2hJCl-N) in Model ClH:NH3 Complexes

EOM-CCSD calculations are performed to evaluate two-bond 35Cl−15N spin−spin coupling constants (2hJCl-N) for ClH:NH3 complexes. Coupling constants for structures in external electric fields of 0.0000, 0.0055, and 0.0150 au are investigated as models for complexes with traditional, proton-shared, and ion-pair hydrogen bonds. Two-dimensional coupling constant surfaces are constructed at these field strengths in the NH and ClH distances, and expectation values, 〈2hJCl-N〉, are calculated for ground and selected excited vibrational states of the dimer- and proton-stretching modes from the corresponding anharmonic wave functions. Single-point values, 2hJCl-N, are also calculated at the equilibrium geometry for each field strength and at the geometry corresponding to the ground-state expectation values of the NH and ClH bond lengths. Coupling constants evaluated in the presence of the electric field are referred to as explicit 〈2hJCl-N〉 and 2hJCl-N. Implicit 〈2hJCl-N〉 and 2hJCl-N are evaluated from the zero-fiel...