Bond Torsion Angles Research Articles

MA'AT Analysis: Unbiased Multi-State Conformational Modeling of Exocyclic Hydroxymethyl Group Conformation in Methyl Aldohexopyranosides.

MA'AT analysis (J. Chem. Inf. Model. 2022, 62, 3135-3141) has been applied to model exocyclic hydroxymethyl group conformation in methyl β-D-glucopyranoside (βGlcOMe), methyl β-D-galactopyranoside (βGalOMe), and methyl β-D-mannopyranoside (βManOMe) in an unbiased manner. Using up to eight NMR J-couplings sensitive to rotation about the C5-C6 bond (torsion angle ω), two-state models of ω were obtained that are qualitatively consistent with the relative populations of the gg, gt, and tg rotamers reported previously. MA'AT analysis gave consistent unbiased gt ⇌ tg models of ω in βGalOMe, with gt more populated than tg and mean values of ω for each population similar to those obtained from aqueous 1-μs MD simulation. Using different combinations of J-couplings had little effect on the βGalOMe model in terms of the mean values of ω and circular standard deviations (CSDs). In contrast, MA'AT analysis of ω in βGlcOMe and βManOMe produced more than one two-state model independent of the ensemble of J-values used in the analyses. These models were characterized by gg ⇌ gt conformer exchange as expected, but the mean values of ω in both conformers varied significantly in the different fits, especially for the gg rotamer. Constrained (biased) MA'AT analyses in which only staggered geometries about ω were allowed gave RMSDs slightly larger than those obtained from the unbiased fits, precluding an assignment of an unbiased model. It is unclear why MA'AT analysis gives consistent and predictable unbiased models of ω in βGalOMe but not in βGlcOMe and βManOMe. One possibility is that the distribution of ω in one or both of the gg and gt conformers in the latter does not conform to a von Mises function (i.e., is not Gaussian-like), but rather to a broad and/or flat distribution that cannot be fit by the current version of MA'AT. Nevertheless, the results of this study provide new evidence of the ability of MA'AT analysis to treat multi-state conformational exchange using only experimental NMR data, extending recent MA'AT applications to furanosyl ring pseudorotation (Biochemistry 2022, 61, 239-251).

Inducing ring distortions in unsubstituted metallophthalocyanines using axial N-heterocyclic carbenes.

A series of metallophthalocyanine (PcM) complexes with axial N-heterocyclic carbene ligands (NHC; 1,3-diisopropylimidazol-2-ylidene (DIP) and 1,3-dimethylbenzimidazol-2-ylidene (DMB)) were prepared and structurally characterized. PcCoII(DIP), PcZnII(DIP), and PcZnII(DMB) are five-coordinate complexes with mild dome-type Pc-ring distortions, while PcFeII(DIP)2 is six-coordinate and has a very large ruffle-type ring-distortion with respect to typical PcM(L)2 systems. The distortion is induced by the highly steric axial DIP ligands. The distortions were quantified and classified by their bond lengths and torsion angles, and according to the normal-coordinate structural decomposition (NSD) analysis. Upon ligation of the NHC, the insoluble PcM materials were solublized in common organic solvents, with typical UV-visible Q-band maxima observable between 658 and 677 nm; the increased solubility is rationalized in terms of the reduced solid-state aggregation of the complexes, attributable to the axial ligation.

Phase-Dependence of Molecular Structures Arising from Weak London Dispersion Interactions.

ConspectusThe structures of molecules can be different in different phases. Intermolecular forces, even those of weak noncovalent interactions (WNCIs), can lead to a preference for quite different conformations in the solid, the gas, and the liquid phases. WNCIs can cause variations in bond lengths, angles, and torsional angles. Since structure is a fundamental concept in chemistry, the knowledge of structural changes with phase is important to understand the source and effects of distorting contributions from WNCIs but also as a predictive tool for the design and stabilization of new bonding situations.X-ray crystallography is ubiquitous and now mostly straightforward to perform, but facilities for the determination of accurate gas-phase structure determination are rare, and gas-phase work is laborious and time-consuming. There are currently about 1.25 million crystal structures and more than 12 500 experimental gas-phase structures, but the intersection of the two data sets that can tell us about the structural differences of the same molecule in different phases is surprisingly small.In this Account, we describe several cases of WNCI-dominated systems for which accurate experimental structure determinations exist for both the gas phase and the solid state and, in one case, also for solution. The examples include aryl-aryl, aryl-alkyl, and alkyl-alkyl interactions; systems with chalcogen and halogen bonding; and fluorine-based interactions in arylboranes. We work out the role of WNCIs in stabilizing large, strained, or sterically overloaded molecules. We will show how flexible molecules will fold under the action of WNCIs when isolated in the gas and how they fold or unfold when they are embedded in an environment of neighbors in crystals. We will show how they can vary in strength when the substitution patterns in aryl groups are changed by different halogens and how intramolecular WNCIs, such as those forming rings, change when such systems experience additional intermolecular WNCIs.Overall, we hope that this Account will give the reader an idea of the type and magnitude of structural changes that can be expected from a free molecule in the gas phase or a single molecule calculated by quantum chemistry compared with one embedded in a crystal. This should define the limits of comparability and provide some predictive concepts of the distortions and variations to be expected.

Crystal structure, Hirshfeld surface analysis, and DFT calculation of 2-pyrazoline-4-ols

DFT computational study at B3LYP/6−31++G** level of theory and single crystal structure analysis of three crystallized 2-pyrazoline-4-ols (DHPZ-OH) were carried out to compare the differences in the structural orientation, electronic and bonding characteristics in both investigations. X-ray crystallography showed that compounds 1, 2, and 3 are crystallized in monoclinic, monoclinic, and triclinic systems, with the space group P21/c, P21/n, and P-1, respectively. Owing to the presence of two neighboring stereocenters, these crystal structures represent the enantiomeric pairs in the crystal packing. Hirshfeld surface analysis (HS) showed that both conventional donor (D)/acceptor (A)-interactions, such as D-H‧‧‧‧‧A, H‧‧‧‧‧N2 and D‧‧‧‧‧O interactions between the appropriate positions in the molecules. Also, the donor/acceptor interaction between the aromatic rings (π-π stacking) ensured the stabilization and orientation of the molecules in the crystal packing. The computational results concerning the geometrical parameters such as the bond lengths, bond torsion, and bond angles were compared with the crystal data. The energies of the HOMO and the LUMO orbitals, and the molecular electrostatic potential (MEP) were generated using the density functional theory (DFT). The MEP map showed the areas of the negative potential region around the electronegative atoms O24 and N2, while the maximum positive sites were around the hydrogen atom attached to O24.

Assessing conformations of small molecules with crystallographic databases

In this work, libraries of crystallographically determined bond lengths, valence angles and torsion angles derived from the Cambridge Structural Database (CSD) are used to generate a score to validate an individual bond length, valence angle or torsion angle. A single score can also be derived for each ensemble of bond lengths, ensemble of valence angles and ensemble of torsion angles in the molecule. The data used to compute the score for the individual parts of the molecule are also used to compute a single value assessing the specific conformation relative to the geometric molecular data contained in the CSD. Such a single-number summary may be used, for example, to validate geometries of modelled molecules or molecular conformations in experimentally or computationally determined crystal structures. An example of how the score can be applied to an individual conformation is provided. Validation of the new score is performed by comparison with existing software for validating molecular geometries (Mogul) and a comparison with the CSD Conformer Generator scores is also carried out. Further application to the experimental standard deviation of C—C bonds of CSD entries is demonstrated, and the Platinum diverse data set is also studied.

A Uranium(II) Arene Complex That Acts as a Uranium(I) Synthon.

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA)3 (2) with KC8 yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA)2] (3) (TDA = N-(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I2 to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA)2)2(μ-I)] (6) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η7-C7H7)(TDA)2(THF)] (7). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA)4] (1-crypt) and U(COT)2, while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA)3]2(μ-η8:η3-C8H8) (8). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA)4 (4) with KC8 gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA)3(THF)] (5). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.

Reactive molecular dynamics simulations of soot formation in acetylene combustion with hydrogen addition

Soot emission from combustion devices not only reduces energy efficiency but also poses a serious risk to global warming and human health. The reactive molecular dynamics simulation is used to study the soot formation process of acetylene as the initial fuel and oxygen as the oxidant, and the chemical effect of hydrogen addition on the soot formation of acetylene combustion is deeply analyzed in this paper. The study found that: in addition to the hydrogen abstraction acetylene addition mechanism, it is also an important path that the long carbon chain aliphatic hydrocarbon with branched chain form polycyclic aromatic hydrocarbons by condensation after continuous dehydrogenation; the chemical effect of hydrogen addition linearly reduces the number of carbon in soot particles, the bond angle energy and torsion angle energy of soot, resulting in a smaller soot particle size and a sparse soot morphology. The shape of the soot gradually changes from a dense spindle-shaped structure to a sparse planar structure; the main chemical effect of hydrogen doping inhibits the dehydrogenation reaction of aliphatic hydrocarbons and aromatic hydrocarbons mainly through H + CaHb↔CaHb−1 + H2, thereby inhibiting the formation of polycyclic aromatic hydrocarbons and soot nucleation, ultimately leading to a reduction in soot production.

CRYSTAL STRUCTURE OF 3-tert-BUTYL- PYRAZOLO[5,1-с][1,2,4]TRIAZINE-3,4-DIYL DICARBOXYLATES

By single crystal X-ray diffraction the structures of (trans-)7-R1-8-methyl-3-tert-butyl-3,4-dihydropyrazolo[5,1-с][1,2,4]triazine-3,4-diyl dibenzoates 3а, 4а (R1 = H, Br) and diacetate 3b (R1 = H) are studied for the first time. The compounds are synthesized by oxidative bromination of respective 1,4-dihydropyrazolo[5,1-c][1,2,4]triazines in the presence of carboxylic acids. Dicarboxylates adopt a twist-conformation with the equatorial (3a) or axial (3b, 4a) orientation of tert-butyl groups relative to the heterocycle. In the case of diacetate 3b, a decrease in the thermal motion of the O4–C16–C17 fragment along with a decrease in the unit cell volume are observed on cooling from 300 K to 200 K. Bond lengths, torsion angles, and molecular packings in the crystals are discussed.

Vfold2D-MC: A Physics-Based Hybrid Model for Predicting RNA Secondary Structure Folding.

Accurate prediction of RNA structure and folding stability has a far-reaching impact on our understanding of RNA functions. Here we develop Vfold2D-MC, a new physics-based model, to predict RNA structure and folding thermodynamics from the sequence. The model employs virtual bond-based coarse-graining of RNA backbone conformation and generates RNA conformations through Monte Carlo sampling of the bond angles and torsional angles of the virtual bonds. Using a coarse-grained statistical potential derived from the known structures, we assign each conformation with a statistical weight. The weighted average over the conformational ensemble gives the entropy and free energy parameters for the hairpin, bulge, and internal loops, and multiway junctions. From the thermodynamic parameters, we predict RNA structures, melting curves, and structural changes from the sequence. Theory-experiment comparisons indicate that Vfold2D-MC not only gives improved structure predictions but also enables the interpretation of thermodynamic results for different RNA structures, including multibranched junctions. This new model sets a promising framework to treat more complicated RNA structures, such as pseudoknotted and intramolecular kissing loops, for which experimental thermodynamic parameters are often unavailable.

Isopropyl 3-deoxy-α-D-ribo-hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside): evaluating trends in structural parameters.

Isopropyl 3-deoxy-α-D-ribo-hexopyranoside (isopropyl 3-deoxy-α-D-glucopyranoside), C9H18O5, (I), crystallizes from a methanol-ethyl acetate solvent mixture at room temperature in a 4C1 chair conformation that is slightly distorted towards the C5SC1 twist-boat form. A comparison of the structural parameters in (I), methyl α-D-glucopyranoside, (II), α-D-glucopyranosyl-(1→4)-D-glucitol (maltitol), (III), and 3-deoxy-α-D-ribo-hexopyranose (3-deoxy-α-D-glucopyranose), (IV), shows that most endocyclic and exocyclic bond lengths, valence bond angles and torsion angles in the aldohexopyranosyl rings are more affected by anomeric configuration, aglycone structure and/or the conformation of exocyclic substituents, such as hydroxymethyl groups, than by monodeoxygenation at C3. The structural effects observed in the crystal structures of (I)-(IV) were confirmed though density functional theory (DFT) calculations in computed structures (I)c-(IV)c. Exocyclic hydroxymethyl groups adopt the gauche-gauche (gg) conformation (H5 anti to O6) in (I) and (III), and the gauche-trans (gt) conformation (C4 anti to O6) in (II) and (IV). The O-glycoside linkage conformations in (I) and (III) resemble those observed in disaccharides containing β-(1→4) linkages.

MICROSTRUCTURE AND THERMPROPERTIES OF N-OCTADECANE DURING PHASE TRANSION: A MOLECULAR DYNAMICS SIMULATION

The thermophysical properties and microstructure of n-octadecane with crystalline and amorphous were investigated by employing the molecular dynamics (MD) simulation. The distribution of the end to end distance and bond torsion angle of the n-octadecane molecular chain and the mean square displacement and thermal conductivity before and after phase transition were also examined. MD simulation results indicates that the molecular chain conformation of amorphous n-octadecane in solid state is gradually changed from stretching to torsion by increasing temperature, and the chains will stretch out as the temperature rises in the liquid state. Compared with amorphous paraffin, the diffusion coefficient and the phase transition temperature of crystalline paraffin is lower than that of amorphous paraffin. The thermal conductivity of crystalline paraffin is much higher than that of amorphous paraffin. It is shown that improving the order degree of PCMs is an effective method to enhance their thermophysical properties.

Arginine off-kilter: guanidinium is not as planar as restraints denote.

Crystallographic refinement of macromolecular structures relies on stereochemical restraints to mitigate the typically poor data-to-parameter ratio. For proteins, each amino acid has a unique set of geometry restraints which represent stereochemical information such as bond lengths, valence angles, torsion angles, dihedrals and planes. It has been shown that the geometry in refined structures can differ significantly from that present in libraries; for example, it was recently reported that the guanidinium moiety in arginine is not symmetric. In this work, the asymmetry of the Nϵ-Cζ-Nη1 and Nϵ-Cζ-Nη2 valence angles in the guanidinium moiety is confirmed. In addition, it was found that the Cδ atom can deviate significantly (more than 20°) from the guanidinium plane. This requires the relaxation of the planar restraint for the Cδ atom, as it otherwise causes the other atoms in the group to compensate by distorting the guanidinium core plane. A new set of restraints for the arginine side chain have therefore been formulated, and are available in the software package Phenix, that take into account the asymmetry of the group and the planar deviation of the Cδ atom. This is an example of the need to regularly revisit the geometric restraint libraries used in macromolecular refinement so that they reflect the best knowledge of the structural chemistry of their components available at the time.

From atoms to bonds, angles and torsions: molecular metrics from crystal space, and two Excel implementations

Values of molecular bond lengths, bond angles and (less frequently) bond torsion angles are readily available from databases, from crystallographic software, and/or from interactive molecular and crystal visualization programs such as Jmol. However, the methods used to calculate these values are less well known. In this paper, the computational methods are described in detail, and live Excel implementations, which permit readers to readily perform the calculations for their own molecular systems, are provided. The methods described apply to both fractional coordinates in crystal space and Cartesian coordinates in Euclidean space (space in which the geometric postulates of Euclid are valid) and are vector/matrix based. In their simplest computational form, they are applied as algebraic expansions which are summed. They are also available in matrix formulations, which are readily manipulated and calculated using the matrix functions of Excel. In particular, their general formulation as metric matrices is introduced. The methods in use are illustrated by a detailed example of the calculations. This contribution provides a significant practical application which can also act as motivation for the study of matrix mathematics with respect to its many uses in chemistry.

Crystal structure determination of N- and O-alkylated tautomers of 1-(2-pyridinyl)-5-hydroxypyrazole

The crystal structure determination of tautomeric products produced by the alkylation of 1-(2-pyridinyl)-3-phenyl-4-propyl-1H-5-hydroxypyrazole 2 was investigated. Treatment of 2 with isopropyloxycarbonyloxymethyl iodide and potassium carbonate under phase-transfer conditions affords two major products out of three possible O-, N-, and C-alkylated tautomers. The tautomeric structures of O-alkylated 3a and N-alkylated 3c were elucidated by means of NMR spectroscopic investigations and confirmed by single crystal X-ray analysis. The single crystal structures of alkylated compounds provide clear difference between the tautomeric pyrazole and pyrazolone ring systems in terms of bond lengths and torsional angles, moreover, conformational changes between two tautomers.

Novel modifications of elemental nitrogen and their molecular structures – a quantumchemical calculation

Using the quantum-chemical calculation methods QCISD and G3, the possibility of the existence of nitrogen molecules with the composition N4, N6, N8 and N10 has been discussed. On the basis of the data obtained, the conclusion about possibility of existence of three novel polymorphic modifications of elemental nitrogen with an even number of atoms in molecules, namely N4 with rectangular and regular tetrahedron shapes, and N6 in a form remotely resembling an open book, has been made. The values of bond lengths, valence and torsion angles, and oscillations frequencies in each of the above-mentioned forms of elemental nitrogen have been presented.

Glycosidic linkage, N-acetyl side-chain, and other structural properties of methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-β-D-mannopyranoside monohydrate and related compounds.

The crystal structure of methyl 2-acetamido-2-deoxy-β-D-glycopyranosyl-(1→4)-β-D-mannopyranoside monohydrate, C15H27NO11·H2O, was determined and its structural properties compared to those in a set of mono- and disaccharides bearing N-acetyl side-chains in βGlcNAc aldohexopyranosyl rings. Valence bond angles and torsion angles in these side chains are relatively uniform, but C-N (amide) and C-O (carbonyl) bond lengths depend on the state of hydrogen bonding to the carbonyl O atom and N-H hydrogen. Relative to N-acetyl side chains devoid of hydrogen bonding, those in which the carbonyl O atom serves as a hydrogen-bond acceptor display elongated C-O and shortened C-N bonds. This behavior is reproduced by density functional theory (DFT) calculations, indicating that the relative contributions of amide resonance forms to experimental C-N and C-O bond lengths depend on the solvation state, leading to expectations that activation barriers to amide cis-trans isomerization will depend on the polarity of the environment. DFT calculations also revealed useful predictive information on the dependencies of inter-residue hydrogen bonding and some bond angles in or proximal to β-(1→4) O-glycosidic linkages on linkage torsion angles φ and ψ. Hypersurfaces correlating φ and ψ with the linkage C-O-C bond angle and total energy are sufficiently similar to render the former a proxy of the latter.

Theoretical study of solvent effects on the hyperpolarizabilities of two chalcone derivatives

The use of organic as nonlinear optical materials has been intensively explored in the recent years due to the ease of manipulation of the molecular structure and the synthetic flexibility regarding the change of substituent groups. In the present work, the linear and nonlinear properties of two chalcones derivatives (E)-1-(4-methylphenyl)-3-phenylprop-2-en-1-one (4MP3P) and (E)-1-(4-Nitrophenyl)-3-phenylprop-2-en-1-one (4NP3P), that differ by the substituent position at the phenyl ring, were studied in the presence of protic and aprotic solvents simulated by the Polarizable Continuum Model (PCM) at DFT/B3LYP/6-311+G(d) level. The static and dynamic (1064 nm) molecular parameters as the dipole moment, linear polarizability, first and second hyperpolarizabilities were studied as function of the solvent dielectric constant value. The geometrical behavior as the chemical bond angles, torsion angles, and partial charges distribution of the compounds were studied, including calculations of gap energies in various solvents. The obtained results revealed that the substituent change of CH3 (4MP3P) to NO2 (4NP3P) benefits the nonlinear optical properties of the compounds in the presence of the solvent media, the absolute values of the parallel first hyperpolarizability were the ones that present the greater variation.

Синтез и рентгендифракционное исследование производных 2,4-диметил-6-оксо-1,6-дигидропиридин-3-карбоксамида

Based on the synthesized starting 2,4-dimethyl-6-oxo-1,6-dihydropyridin-3-carboxamide crystallizing as a dihydrate, 9 derivatives of it including O- and N-phenacyl substituted products were obtained through original rearrangements and cyclizations. The corresponding oxazolopyridinium perchlorate was obtained from the latter preparation in the presence of acid, from which indolysin was obtained under the action of a nitrogenous base. The obtained molecular and crystal structures of all the compounds were studied by single crystal diffraction: 2,4-dimethyl-6-oxo-1,6-dihydropyridine-3-carboxamide dihydrate, 6-hydroxy-2,4-dimethyl-3-carbamoyl-pyridinium chloride monohydrate, 2,4-dimethyl-6-methoxypyridine-3-carboxamide, 1-[2-(4-methylphenyl)-2-oxoethyl]-2,4-dimethyl-6-oxo-1,6-dihydropyridine-3-carboxamide, 2,4-dimethyl-6-oxo-1-[2- (4-[chlorophenyl)-2-oxoethyl]-1,6-dihydropyridine-3-carboxamide, 2,4-dimethyl-6-[2-(4-methylphenyl)-2-oxo-ethoxy]pyridin-3-carbox-amide, 2,4-dimethyl-6-[2-(4-chlorophenyl)-2-oxoethoxy] pyridin-3-carb xamide, 6-carbamoyl-2-(4-chlorophenyl)-5,7-dimethyl[1,3]oxazolo[3,2-a]pyridin-4 perchlorate, 7-methyl-5-morpholin-4 -yl-2-(4-chlorophenyl) indolysin-8-carboxamide. A structural analysis of the by-products of 1,4-bis (4-methylphenyl) butane-1,4-dione and 1,4-bis(4-chlorophenyl) butane-1,4-dione was also carried out. The synthesis methods of each of them are given. A comparative analysis of bond lengths, valence and torsion angles in similar fragments of molecules is carried out. Hydrogen bonds were revealed in the structures and their influence on the strength of molecular packing in crystals was shown. All structures of the studied compounds, in addition to XRD, are characterized by 1H NMR spectra.

C-N Bond Rotation Controls Photoinduced Electron Transfer in an Aminostyrene-Stilbene Donor-Acceptor System.

We investigate energy transfer and electron transfer in a dimethylsilylene-spaced aminostyrene-stilbene donor-acceptor dimer using time-dependent density functional theory calculations. Our results confirm that the vertical S3, S2, and S1 excited states are, respectively, a local excitation on the aminostyrene, local excitation on the stilbene, and the charge-transferred (CT) excited state with electron transfer from aminostyrene to stilbene. In addition, an energy minimum with the C-N bond of the amino group twisted at about 90° is also identified on the S1 potential energy surface. This S1 state exhibits a twisted intramolecular charge transfer (TICT) character. A potential energy scan along the C-N bond torsional angle reveals a conical intersection between the S2 stilbene local excitation and the S1 CT/TICT state at a torsional angle of ∼60°. We thus propose that the conical intersection dominates the electron transfer dynamics in the donor-acceptor dimer and copolymers alike, and the energy barrier along the C-N bond rotation controls the efficiency of such a process. Moreover, we show that despite the zero oscillator strength of the S1 excited states in the CT and TICT minima, an emissive S1 state with a V-shaped conformational structure can be located. The energy of this V-shape CT structure is thermally accessible; therefore, it is expected to be responsible for the CT emission band of the dimer observed in polar solvents. Our data provide a clear explanation of the complex solvent-dependent dual emission and photoinduced electron transfer properties observed experimentally in the dimer and copolymer systems. More importantly, the identifications of the conical intersection and energy barrier along the C-N bond rotation provide a novel synthetic route for controlling emissive properties and electron transfer dynamics in similar systems, which might be useful in the design of novel organic optoelectronic materials.

Competing Intramolecular Hydrogen Bond Strengths and Intermolecular Interactions in the 4-Aminobutanol-Water Complex.

We seek to determine the effect of competing intermolecular hydrogen bonds from water on the preferred conformation of 4-aminobutanol (4AB) monomers stabilized by intramolecular hydrogen bonds. Toward this end, the rotational spectrum of the 4-aminobutanol-H2O complex was recorded using Fourier transform microwave spectroscopy and fit to the rotational, quadrupole coupling, and centrifugal distortion constants of the Watson S-reduction Hamiltonian. The experimental results are consistent with a 4AB-water complex that preserves the intramolecular hydrogen bond within the 4AB monomer and forms a single intermolecular bond with water acting as a donor. The experimental monomer structure agrees well with the lowest energy conformation calculated at the MP2/6-311++G(d,p) level of theory. Upon complex formation and the introduction of competing intermolecular bonds from water, only small changes in the OH···N intramolecular hydrogen bond and backbone torsional angles of the 4-aminobutanol monomer are observed. Similar small changes were observed for the shorter chain 3-aminopropanol amino alcohol monomer when complexed with water, in contrast to the 2-aminoethanol-H2O complex. In the latter, a large change in the backbone torsional angle and a breaking of the intramolecular hydrogen bond were observed. Thus, extending the methylene chain results in an increase in the strength of the intramolecular hydrogen bond in unbranched amino alcohols.