Differences In Bond Lengths Research Articles

Single-Crystal Growth and Characterization of Disilver(I) Monofluorophosphate(V), Ag2PO3F: Crystal Structure, Thermal Behavior, Vibrational Spectroscopy, and Solid-State19F,31P, and109Ag MAS NMR Spectroscopy

Single crystals of disilver(I) monofluorophosphate(V), Ag2PO3F (1), were obtained by slow evaporation of a diluted aqueous Ag2PO3F solution. Compound 1 adopts a new structure type and crystallizes in the monoclinic space group C2/c with eight formula units and lattice parameters of a = 9.2456(8) A, b = 5.5854(5) A, c = 14.7840(13) A, and beta = 90.178(2) degrees. The crystal structure of 1 [R(F2 > 2sigma(F2) = 0.0268, wR(F2 all) = 0.0665] is composed of three crystallographically independent Ag+ cations and PO3F2- anions as single building units. The oxygen environment around each of the Ag+ cations is different, with one Ag+ in distorted octahedral (d(Ag-O) = 2.553 A), one in nearly rectangular (d(Ag-O) = 2.445 A), and one in distorted tetrahedral (d(Ag-O) = 2.399 A) coordination. Additional Ag-F contacts to more remote F atoms located at distances >2.80 A augment the coordination polyhedra for the two latter Ag+ cations. The monofluorophosphate anion deviates slightly from C3v symmetry and exhibits the characteristic differences in bond lengths, with a mean of 1.510 A for the P-O bonds and one considerably longer P-F bond of 1.575(2) A. Compound 1 was further characterized by vibrational spectroscopy (Raman and IR) and solid-state 19F, 31P, and 109Ag MAS NMR spectroscopy. The value for the isotropic one-bond P-F coupling constant in 1 is 1JPF = -1045 Hz. Thermal analysis (TG, DSC) revealed a reversible phase transition at 308 degrees C, which is very close to the decomposition range of 1. Under release of POF3, Ag4P2O7 and Ag3PO4 are the thermal decomposition products at temperatures above 450 degrees C.

X-ray absorption of Zn1−xCoxO thin films: A local structure study

In this study, the application of synchrotron radiation microprobe to the analysis of Co incorporation in Zn1−xCoxO is reported. From the Co and Zn fluorescence line intensity ratio, the Co concentrations were deduced. A combination of fluorescence mapping with x-ray absorption spectroscopic techniques made possible to examine not only the uniform elemental distribution but also the short range structural order with the Co content. In Zn1−xCoxO films grown by pulsed laser depositions the Co atoms are in tetrahedral site configuration, corresponding to a substitutional site in wurtzite structure. Within the experimental accuracy, no significant bond length deviation from the pure ZnO structure was observed in the considered compositional range.

Total carbo-Mer of Benzene, Its carbo-Trannulene Form, and the Zigzag Nanotube Thereof

The total carbo-mer of benzene, hexaethynyl carbo-benzene C30H6, has been calculated at the B3PW91/6-31G** level. Its geometrical and magnetic characteristics are compared with those of the C18H6 partial carbo-mers, unsubstituted carbo-benzene, and hexaethynylbenzene. The carbo-[6]trannulene isomer is found to exist as a minimum on the singlet spin state potential energy surface (PES) and is 65.6 kcal.mol(-1) higher in energy than hexaethynyl-carbo-benzene. In the former, a strong cyclic electron delocalization is evidenced from the root-mean-square deviation (rms) of the ring bond lengths and the NICS value computed at the centroid of the trannulene ring. As an alternative to the graphene sheet wrapping process traditionally used to illustrate the construction of carbon nanotubes, a dehydrocoupling-stacking process is invoked for the construction of zigzag nanotubes from trannulene bricks. The process is applied to the carbo-[6]trannulene brick to generate a novel type of acetylene-expanded carbon nanotube, which is a polymer of primitive C60 segments. A C60H6 carbo-meric equivalent of a cyclacene belt is first considered. Two such segments are then formally dehydrocoupled to generate a cylindrical (C60)2H6 molecule, the central part of which is assumed to be a relevant model for the infinite nanotube. Axial and sectional electron delocalization inside the tube models is discussed on the basis of bond length analysis, NICS values, pi MO analysis, and singlet-triplet state energy gap. The capping of the C120 cyclinder is finally addressed by use of carbo-[3]radialenic units.

A theoretical investigation of steric effects on 1H chemical shifts of camphor and norcamphor derivatives

Camphor and norcamphor derivatives provide convenient models for testing and evaluating some of the results of our work on substituted adamantanes and for checking their applicability to other systems. Our results show that the reduction in C–H bond lengths associated with steric interactions are not as pronounced in systems that are less rigid. Deshielding of 1H chemical shifts on introduction of the substituent in exo and endo positions can also be traced to angular distortions which are stronger in norcamphor, bornane and camphor than in norbornane due to additional steric interactions with a carbonyl group, with methyl groups, or with both. 1H chemical shifts are sensitive probes for steric effects, reflecting differences in bond lengths, bond angles and dihedral angles in a systematic way. Given the proper models for comparison purposes they can be used to evaluate the size of a certain group and locate its interactions with the rest of the molecular framework.

Structure of zirconium butoxide complexes in n-butanol solutions

EXAFS and SAXS were used for structure elucidation of zirconium butoxide complexes in n-butanol at concentrations from 0.3 g to 0.015 g ZrO2 in 1 ml. The basic structural unit of the complex is a tetramer. It has two equal sides with zirconium atoms linked by double oxygen bridges and with zirconium-zirconium distances of 3.5 A. The other sides in the tetramer are 3.3 A and 3.9 A. This difference in bond lengths is explained by the different numbers of double or single ligand bridges between zirconium atoms. The tetramers are apt to undergo oligomerization to form particles with a diameter of 80 A in solution.

Coordination polyhedra LnFn (Ln = La-Lu) in crystal structures

Voronoi-Dirichlet (VD) polyhedra and the intersecting spheres method are used to study stereochemical features of lanthanides (Ln) surrounded by fluorine atoms in the structures of 118 compounds. The coordination numbers of Ln atoms range from 6 to 12. Nine types of coordination polyhedra LnFn are found; depending on the CN of Ln, the Ln-F bond length can change by 0.2–0.7 A. Despite the considerable differences in bond lengths, the volume of the VD polyhedra of Ln atoms depends only on their nature and oxidation state. The change in the radii of the spherical domains whose volume is equal to that of the VD polyhedra of Ln atoms shows the tetrad effect.

DFT modeling and spectroscopic study of metal–ligand bonding in La(III) complex of coumarin-3-carboxylic acid

The binding mode of coumarin-3-carboxylic acid (HCCA) to La(III) is elucidated at experimental and theoretical level. The complexation ability of the deprotonated ligand (CCA −) to La(III) is studied using elemental analysis, DTA and TGA data as well as FTIR, 1H NMR and 13C NMR spectra. The experimental data suggest the complex formula La(CCA) 2(NO 3)(H 2O) 2. B3LYP, BHLYP, B3P86, B3PW91, PW91P86 and MPW1PW91 functionals are tested for geometry and frequency calculations of the neutral ligand and all of them show bond length deviations bellow 1%. B3LYP/6-31G(d) level combined with large quasi-relativistic effective core potential for lanthanum is selected to describe the molecular, electronic and vibrational structures as well as the conformational behavior of HCCA, CCA − and La–CCA complex. The metal–ligand binding mode is predicted through molecular modeling and energy estimation of different La–CCA structures. The calculated atomic charges and the bonding orbital polarizations point to strong ionic metal–ligand bonding in La–CCA complex and insignificant donor acceptor interaction. Detailed vibrational analysis of HCCA, CCA − and La(CCA) 2(NO 3)(H 2O) 2 systems based on both calculated and experimental frequencies confirms the suggested metal–ligand binding mode.

Modeling Misfit Dislocation Arrays for the Growth of Low-Defect Density AlSb on Si

ABSTRACTWe present analytical models and experimental results to describe low-defect density growth (∼ 6 × 105/cm2) of highly mismatched antimonides on Si and GaAs substrates, with strain relief achieved at the growth interface through periodic, 90° interfacial misfit dislocations (IMF). We use molecular mechanics (MM) based modeling techniques to understand, at the atomic level, the spontaneous formation and energetics of these IMF. We have modeled, grown and characterized two systems extensively, these are - AlSb on Si with ∼ 13% mismatch and GaSb on GaAs with 7.83% lattice mismatch. Growth of these materials by molecular beam epitaxy (MBE) and subsequent High-Resolution Transmission Electron Microscopy (HR-TEM) has indicated that there is no tetragonal distortion in these two systems despite the high lattice mismatch. Instead, the mismatched epi-layers spontaneously form periodic IMF arrays that run along both [110] and [1-10] directions and relieve almost 100% of the strain in a few monolayers of deposition. To model this form of strain relief, we use existing theories of strain relief adapted for very high strain conditions and we also use bond energetics to model the strain-relieving interface. The IMFs in these systems are periodic and so is the deviation in bond lengths and bond angles, which restricts our calculation space to a finite number of elements. We shall also demonstrate extensive growth and characterization results of the materials grown with a particular emphasis on the strain-relieving interface to show excellent agreement of the experimental data with the proposed models. The high quality and low-defect density in AlSb grown on Si, has helped us demonstrate optically pumped IR VCSELs and edge emitters monolithically on Si (001) and this data will also be presented.

The crystallographic phase transition for a ferric thiosemicarbazone spin crossover complex studied by X-ray powder diffraction

The crystal structure of a spin-transition compound, namely the thiosemicarbazone ferric complex Li[Fe(5BrThsa)2]·H2O, was solved from powder X-ray diffraction data at temperatures where the high-spin (373 K) and low-spin (150 K) phases prevail. The methodology is based on traditional approaches (direct methods) combined with direct space strategy. Both phases crystallise in the monoclinic system P21/c. At 373 K, the characteristics of the [FeN2O2S2] coordination core are consistent with those reported for high-spin iron(III) thiosemicarbazone complexes: a distorted coordination polyhedron and non-equivalent metal–ligand bond lengths. When the temperature is reduced to 150 K, a decrease of the β angle from ca. 101° (373 K) to ca. 90° (150 K) is the only major modification of the cell parameters. The low-spin molecular structure reveals significant differences in bond lengths and bond angles compared to the high-spin structure. Finally, an extended hydrogen-bond network is implicated in the cooperative phase transition, as supported by strong intermolecular contacts between the ferric complexes and the water molecules and the crystallographic phase transition is associated with pronounced lattice reorganization.

Structural and Functional Evidence for Bacillus subtilis PaiA as a Novel N1-Spermidine/Spermine Acetyltransferase

Bacillus subtilis PaiA has been implicated in the negative control of sporulation as well as production of degradative enzymes. PaiA shares recognizable sequence homology with N-acetyltransferases, including those that can acetylate spermidine/spermine substrates. We have determined the crystal structure of PaiA in complex with CoA at 1.9 A resolution and found that PaiA is a member of the N-acetyltransferase superfamily of enzymes. Unexpectedly, we observed the binding of an oxidized CoA dimer in the active site of PaiA, and the structural information suggests the substrates of the enzyme could be linear, positively charged compounds. Our biochemical characterization is also consistent with this possibility, since purified PaiA possesses N1-acetyltransferase activity toward polyamine substrates including spermidine and spermine. Further, conditional overexpression of PaiA in bacteria results in increased acetylation of endogenous spermidine pools. Thus, our structural and biochemical analyses indicate that PaiA is a novel N-acetyltransferase capable of acetylating both spermidine and spermine. In this way, the pai operon may function in regulating intracellular polyamine concentrations and/or binding capabilities. In addition to preventing toxicity due to polyamine excess, this function may also serve to regulate expression of certain bacterial gene products such as those involved in sporulation.

Absorption Spectra of Ground-State and Low-Lying Electronic States of Copper Nitrosyl: A Rare Gas Matrix Isolation Study

The reaction of ground-state Cu atoms with NO during condensation in solid argon, neon, and binary argon/neon mixtures has been reinvestigated. In addition to the ground-state already characterized in rare gas matrixes by its nu1 mode in reactions of laser-ablated Cu with nitric oxide, another very low lying electronic state is observed for CuNO in solid argon. Photoconversion and equilibrium processes are observed between the two lowest lying electronic states following photoexcitations to second and third excited states in the visible and near-infrared. The electronic spectrum of the CuNO complex was also recorded to understand the photoconversion processes. In solid neon, only the ground state (probably 1A') and the second and third excited states are observed. This suggests that interaction with the argon cage stabilizes the triplet state to make 1A' and 3A' ' states almost isoenergetic in solid argon. On the basis of previous predictions founded on DFT calculations on the very low lying 1A' and 3A' ', a mechanism is proposed, involving the singlet-triplet state manifolds. For these two lower and one higher electronic states, 14N/15N, 16O/18O, and 63Cu/65Cu isotopic data on nu1, nu2, and nu3 have been measured. On the basis of harmonic force-field calculations and relative intensities in the vibronic progressions, some structural parameters are estimated. The molecule is bent in all electronic states, with Cu-N-O bond angles varying slightly around 130 +/- 10 degrees , but the Cu-N bond force constants are substantially different, denoting larger differences in bond lengths.

Three New Cytotoxic ent‐Kaurane Diterpenoids from Isodon weisiensis C. Y. Wu

AbstractThree new cytotoxic ent‐kaurane diterpenoids, (1α,7α,14β)‐1,7,14‐trihydroxy‐ent‐kaur‐16‐en‐15,18‐dione (1), (1α,7α,14β)‐1,7,14,18,20‐pentahydroxy‐ent‐kaur‐16‐en‐15‐one (2), and (3β,7α,14β)‐3,7,14‐tris(acetyloxy)‐ent‐kaur‐16‐en‐15‐one (3), were isolated from Isodon weisiensis C. Y. Wu. Their structures were elucidated by spectroscopic methods, including 2D‐NMR techniques, and the crystal structure of 1 was determined by single‐crystal X‐ray‐diffraction analysis. The chosen crystal of 1 was orthorhombic, space group P212121, and there were two molecules with little difference in bond length and bond angle in the least‐asymmetry unit. Compounds 1–3 showed significant cytotoxic activities against human‐cancer cell lines Bel‐7402 and HO‐8910.

Ultrafast and Low Barrier Motions in the Photoreactions of the Green Fluorescent Protein

Green fluorescent protein (GFP) fluoresces efficiently under blue excitation despite major electrostatic rearrangements resulting from photoionization of the chromophore and neutralization of Glu-222. A competing phototransformation process, which ionizes the chromophore and decarboxylates Glu-222, mimics the electrostatic and structural changes in the fluorescence photocycle. Structural and spectroscopic analysis of the cryogenically stabilized photoproduct at 100 K and a structurally annealed intermediate of the phototransformed protein at 170 K reveals distinct structural relaxations involving protein, chromophore, solvent, and photogenerated CO2. Strong structural changes of the 100 K photoproduct after decarboxylation appear exclusively within 15 angstroms of the chromophore and include the electrostatically driven perturbations of Gln-69, Cys-70, and water molecules in an H-bonding network connecting the chromophore. X-ray crystallography to 1.85 angstroms resolution and static and picosecond time-resolved IR spectroscopy identify structural mechanisms common to phototransformation and to the fluorescence photocycle. In particular, the appearance of a 1697 cm(-1) (+) difference band in both photocycle and phototransformation intermediates is a spectroscopic signature for the structural perturbation of Gln-69. This is taken as evidence for an electrostatically driven dynamic response that is common to both photoreaction pathways. The interactions between the chromophore and the perturbed residues and solvent are decreased or removed in the T203H single and T203H/Q69L double mutants, resulting in a strong reduction of the fluorescence quantum yield. This suggests that the electrostatic response to the transient formation of a buried charge in the wild type is important for the bright fluorescence.

Electronic Structure and Electron Transport Characteristics of a Cobalt Complex

The molecular and electronic structures and electron transport characteristics of a Co complex are investigated using first principles calculations. The Co complex belongs to the D(2d) point group, and its two ligands are perpendicular to each other. The central atom Co forms a distorted octahedron with six donor N atoms. In a low oxidation state, the bond length between Co and pyrrole nitrogen, 1.849 A, is much shorter than the distance between Co and pyridine nitrogen, 2.168 A, while, in a high oxidation state, the bond length differences between Co and pyrrole nitrogen, 1.814 A, and between Co and pyridine nitrogen, 1.990 A, are not as large as those in the Co2+ complex. The HOMO energy of the low oxidation state is very close to the Fermi level of bulk Au, allowing hole creation in the molecule. On the other hand, the LUMO energy of the high oxidation state is close to the Au Fermi level, allowing a low barrier for electron injection from the Au cathode to the molecule. These structural characteristics make the Co complex a good hole-conduction molecule. The density of states, transmission probability, and I-V characteristics are evaluated using the Green function approach.

Assessment of Density Functionals for the High‐Spin/Low‐Spin Energy Difference in the Low‐Spin Iron(II) Tris(2,2′‐bipyridine) Complex

In the iron(II) low-spin complex [Fe(bpy)3]2+, the zero-point energy difference between the 5T2g(t4(2g)e2g) high-spin and the 1A(1g)(t(6)2g) low-spin states, Delta(E)0HL, is estimated to lie in the range of 2500-5000 cm(-1). This estimate is based on the low-temperature dynamics of the high-spin-->low-spin relaxation following the light-induced population of the high-spin state and on the assumption that the bond-length difference between the two states Delta(r)HL is equal to the average value of approximately 0.2 A, as found experimentally for the spin-crossover system. Calculations based on density functional theory (DFT) validate the structural assumption insofar as the low-spin-state optimised geometries are found to be in very good agreement with the experimental X-ray structure of the complex and the predicted high-spin geometries are all very close to one another for a whole series of common GGA (PB86, PW91, PBE, RPBE) and hybrid (B3LYP, B3LYP*, PBE1PBE) functionals. This confirmation of the structural assumption underlying the estimation of Delta(E)0HL from experimental relaxation rate constants permits us to use this value to assess the ability of the density functionals for the calculation of the energy difference between the HS and LS states. Since the different functionals give values from -1000 to 12000 cm(-1), the comparison of the calculated values with the experimental estimate thus provides a stringent criterion for the performance of a given functional. Based on this comparison the RPBE and B3LYP* functionals give the best agreement with experiment.

Isolation and Characterization of Phenyl Viologen as a Radical Cation and Neutral Molecule

The chemical synthesis, isolation, and characterization of phenyl viologen (PV) as a dication, radical cation, and neutral species are described. Single-crystal X-ray diffraction of PV(2+)2Cl(-.)2H2O and PV(.+)PF(6)(-).pyridine reveals the expected differences in bond lengths and also a structural change from two coplanar central rings in PV(.+) to a twist of 36 degrees between the two central rings in PV(2+). The phenyl viologen radical cation exhibits behavior characteristic of many radical cations, including weak pi-dimerization in the solid state and reversible pi-dimerization in solution. Electrical conductivity measurements of neutral phenyl viologen, the first such measurements of a neutral viologen, reveal that it is a significantly better conductor than the radical cation. Differences in geometric relaxation during charge transfer offer a possible explanation for the higher conductivity of the neutral viologen.

Exceptionally Large Difference in Bond Length among Conformational Isomorphs of a Hexaphenylethane Derivative with a Dispiropyracene Skeleton

A large difference of 0.064(3) Å for a chemically equivalent Csp3−Csp3 bond was observed among crystallographically independent molecules with different conformations (conformational isomorphs) of a hexaphenylethane derivative, dispiro[(10-methylacridan)-9,1‘-pyracene-2‘,9‘ ‘-(10-methylacridan)] (1): triclinic, P1̄, a = 17.699(3) Å, b = 18.111(3) Å, c = 18.844(3) Å, α = 111.183(2)°, β = 93.828(1)°, γ = 102.130(2)°, V = 5438.5(14) Å3, T = −180 °C, Z = 8 (four independent molecules: A, A‘, B, and B‘). While B and B‘ adopt the skewed conformation to reduce steric repulsion in 1, in other positions of the crystal, molecules A and A‘ adopt the eclipsed conformation, which induces a much longer bond length [1.771(3) and 1.758(3) Å versus 1.712(2) and 1.707(2) Å].

Complexes of Functionalized Dipyrido[3,2-a:2‘,3‘-c]-phenazine: A Synthetic, Spectroscopic, Structural, and Density Functional Theory Study

The ligands 11-bromodipyrido[3,2-a:2',3'-c]phenazine and ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate have been prepared and coordinated to ruthenium(II), rhenium(I), and copper(I) metal centers. The electronic effects of substitution of dipyrido[2,3-a:3',2'-c]phenazine (dppz) have been investigated by spectroscopy and electrochemistry, and some photophysical properties have been studied. The crystal structures of [Re(L)(CO)(3)Cl] (L = ethyl dipyrido[3,2-a:2',3'-c]phenazine-11-carboxylate or 11-bromodipyrido[3,2-a:2',3'-c]phenazine) are presented. Density functional theory calculations on the complexes show only small deviations in bond lengths and angles (most bonds within 0.02 Angstroms, most angles within 2 degrees) from the crystallographic data. Furthermore, the vibrational spectra of the strongest Raman and IR bands are predicted to within an average 6 cm(-1) for the complexes [Re(L)(CO)(3)Cl] and [Cu(L)(triphenylphosphine)(2)]BF(4) (in the 1000-1700 cm(-1) region). Spectroscopic and electrochemical evidence suggest that reduction of the complex causes structural changes across the entire dppz ligand. This is unusual as dppz-based ligands typically have electrochemical properties that suggest charge localization with reduction on the phenazine portion of the ligand. The excited-state lifetimes of the complexes have been measured, and they range from ca. 200 ns for the [Ru(L)(2,2'-bipyridine)(2)](PF(6))(2) complexes to over 2 mus for [Cu(11-bromodipyrido[3,2-a:2',3'-c]phenazine)(PPh(3))(2)](BF(4)) at room temperature. The emission spectra suggest that the unusually long-lived excited states of the copper complexes result from metal-to-ligand charge transfer (MLCT) transitions as they are completely quenched in methanol. Electroluminescent films may be fabricated from these compounds; they show MLCT state emission even at low doping levels [<0.1% by weight in poly(vinylcarbazole) polymer matrix].

Benzoyl Transfer Reactivities of Racemic 2,4‐Di‐O‐acyl‐myo‐inosityl 1,3,5‐Orthoesters in the Solid State: Molecular Packing and Intermolecular Interactions Correlate with the Ease of the Reaction

Racemic 2,4-di-O-acyl-myo-inosityl 1,3,5-orthoesters undergo transesterification catalyzed by sodium carbonate with varying ease of reaction in the solid state; reactions in solution and melt do not show such varied differences. An interesting crystal of a 1:1 molecular complex of highly reactive racemic 2,4-di-O-benzoyl-myo-inosityl 1,3,5-orthoformate and its orthoacetate analogue exhibited better reactivity than the latter component alone. Single-crystal X-ray structures of the reactants have been correlated with the observed differences in the acyl-transfer efficiencies in the solid state. Although each of the derivatives helically self-assembles around the crystallographic 2(1) axis linked through O-H...O hydrogen bonding, the pre-organization of the reactive groups (C=O [El] and OH [Nu]), C-H...O and the C-H...pi interactions are significantly more favourable for the reactive derivatives than the less reactive ones. Bond-length distributions also showed differences; the O-C bond of the axial benzoyl group, which gets cleaved during the reaction, is longer (1.345-1.361 A) relative to the chemically equivalent O-C bond of the equatorial benzoyl group (1.316-1.344 A) in the reactive derivatives. These bond-length differences are not significant in the less reactive derivatives. The overall molecular organization is different too; the strikingly discrete helices, which may be viewed as "reaction tunnels" and are held by interhelical interactions, are clearly evident in the reactive derivatives in comparison with the less reactive ones.

Refinement of protein crystal structures using energy restraints derived from linear-scaling quantum mechanics

A novel method is proposed in which combined restraints derived from linear-scaling semiempirical quantum-mechanical (QM) calculations and X-ray diffraction data are combined to refine crystal structures of proteins. Its performance has been tested on a small protein molecule, bovine pancreatic trypsin inhibitor (BPTI). The refinement involves minimization of the sum of a geometric energy function and an X-ray target function based on either the least-squares residual or the maximum-likelihood formalism. For comparison, similar refinement runs have also been performed using energy restraints derived from the force field available in the Crystallography & NMR System (CNS) program. The QM refinements were carried out with weights that were varied by several orders of magnitude and the optimal weights were identified by observing the trend in the final free R values, QM heats of formation and coordinate root-mean-square deviations (r.m.s.d.s) from the crystal structure. It is found that the QM weights are typically smaller but generally on the same scale as the molecular-mechanics (MM) weights for the respective X-ray target functions. The crystallographic R, free R, real-space R values and correlation coefficients based on the structures refined with the energy restraints derived from our QM calculations and Engh and Huber parameters are comparable, suggesting that the QM restraints are capable of maintaining reasonable stereochemistry to a similar degree as the force-field parameters. A detailed inspection of the structures refined with the QM and MM energy restraints reveals that one of the common differences between them and the crystal structure is that the strained bond angles in the crystal structure are corrected after energetically restrained refinements. Systematic differences in certain bond lengths between the QM-refined structures and the statistical averages of experimental structures have also been observed and discussed.