Experimental evidence for intramolecular blue-shifting C-H...O hydrogen bonding by matrix-isolation infrared spectroscopy

Experimental evidence for intramolecular blue-shifting C-H...O hydrogen bonding is presented. Argon matrix-isolation infrared spectra of 1-methoxy-2-(dimethylamino)ethane exhibit a band at 3016.5 cm-1. Spectral behavior with annealing indicates that this band is assigned to the most stable conformer, trans-gauche-(trans|gauche'), with an intramolecular C-H...O hydrogen bond. Density functional calculations show that this band arises from the stretching vibration of the C-H bond participating in the formation of the C-H...O hydrogen bond. The C-H bond is shortened by 0.004 A, and the C-H stretching band is blue-shifted by at least 35 cm-1 on the formation of the hydrogen bond. The (C)H...O distance is calculated as 2.38 A, which is shorter than the corresponding van der Waals separation by 0.3 A.     

Bifurcated hydrogen-bonding effect on the shielding and coupling constants in trifluoroacetyl pyrroles as studied by 1H, 13C and 15N NMR spectroscopy and DFT calculations

According to the (1)H, (13)C and (15)N NMR spectroscopic data and DFT calculations, bifurcated N--H...N and N--H...O intramolecular hydrogen bond is shown to be present in 2-trifluoroacetyl-5-(2'-pyridyl)-pyrrole. This bifurcated hydrogen bond causes an increase in the absolute size of the (1)J(N,H) coupling constant by about 6 Hz, and the deshielding of the bridge proton by 2 ppm. DFT calculations show that the influence of the N--H...N and N--H...O intramolecular hydrogen bonds on the (1)J(N,H) coupling and proton shielding is almost additive, although the components of the bifurcated hydrogen bond slightly weaken each other. In 2-trifluoroacetyl-5-(2'-pyridyl)-pyrrole, the coupling constants involving the fluorine and the N--H covalent bond nuclei depend dramatically on the spatial position of the pyridine ring. The pyridine ring rotation operates as a quantum switch controlling the spin information transfer between the (19)F and (15)N nuclei, as well as the proton.     

Comparative analysis of hydrogen bonding with participation of the nitrogen, oxygen and sulfur atoms in the 2(2'-heteroaryl)pyrroles and their trifluoroacetyl derivatives based on the 1H, 13C, 15N spectroscopy and DFT calculations

The N-H...X (X = N,O,S) intramolecular hydrogen bond in the series of 2(2'-heteroaryl)pyrroles and their trifluoroacetyl derivatives is examined by the (1)H, (13)C, (15)N spectroscopy and density functional theory (DFT) calculations. The influence of the hydrogen bond on coupling and shielding constants is considered. It is shown that the N-H...N intramolecular hydrogen bond causes a larger increase in the absolute size of the (1)J(N,H) coupling constant and a larger deshielding of the bridge proton than the N-H...O hydrogen bond. The effect of the N-H...S interaction on the (1)J(N,H) coupling constant and the shielding of the bridge proton is small. The NMR parameter changes in the series of the 2(2'-heteroaryl)pyrroles due to N-H...X hydrogen bond and the series of the 1-vinyl-2-(2'-heteroaryl)-pyrroles due to C-H...X hydrogen bond have the same order. The proximity of the nitrogen, oxygen or sulfur lone pair to the F...H hydrogen bridge quenches the trans-hydrogen bond spin-spin couplings (1h)J(F,H-1) and (2h)J(F,N).     

Beta-alanine-hydrochloride (2:1) crystal: structure, 13C NMR and vibrational properties,protonation character

The crystal structure of beta-alanine-hydrochloride (2:1) complex (2A-HCl) has been determined by X-ray diffraction method at 298 and 100 K as monoclinic, space group C2/c, Z=4. The crystal comprises chloride anions and protonated beta-alanine dimers: two beta-alanine zwitterions are joined by strong, symmetric (Ci) hydrogen bond with the O...O distance of 2.473 A at room temperature. Powder FT-IR and FT-Raman as well as solid state 13C NMR spectra provide insights into the solid structure of this complex, character of its hydrogen bonds and the beta-alanine protonation.     

Effects of ionization on N-glycylglycine peptide: influence of intramolecular hydrogen bonds

The ionization effects on 28 conformations of N-glycylglycine are analyzed by means of the hybrid B3LYP and the hybrid meta-MPWB1K density functionals and by single-point calculations at the CCSD(T) level of theory. The most favorable process observed corresponds to the ionization of the only neutral conformation that presents a OH...NH2 intramolecular hydrogen bond, which leads to CO2 elimination after a spontaneous proton transfer from -COOH to NH2. The remaining neutral structures evolve to 20 different conformations of N-glycylglycine radical cation, which lie about 25-40 kcal/mol higher than the decarboxylated [NH3CH2CONHCH2]+*...[CO2] complex. Structural changes induced by ionization depend on the intramolecular hydrogen bonds of the initial conformation, since they determine the nature of the electron hole formed. In most cases, ionization takes place at the terminal -NH2 and -CO of the amide bond, which produces a strengthening of the peptide bond and the formation of new -NH2...OC(amide) and -NH2...OCOH hydrogen bonds. However, if -NH2 and -CO(amide) simultaneously act as proton acceptor in the neutral conformation, ionization is mainly localized at the carboxylic group, which produces a strengthening of the -COOH...OC(amide) bond. Both functionals lead to similar trends and compare well with CCSD(T) results except for a few cases for which B3LYP provides a too delocalized picture of the electron hole and consequently leads to artificial geometry reorganization.     

The first solid enols of anhydrides. Structure, properties, and enol/anhydride equilibria(1)

Reaction of beta-methylglutaconic anhydride with NaOMe followed by reaction with methyl or phenyl chloroformate gave the corresponding O-methoxy (and O-phenoxy) carbonylation derivatives. Reaction of the anhydride with MgCl2/pyridine, followed by methyl chloroformate gave C-methoxycarbonylation at C3 of the anhydride. The product (4) was previously suggested by calculation to be the enol of the anhydride 5 and this is confirmed by X-ray crystallography (bond lengths: C-OH, 1.297 A; C1C2 1.388 A; HO...O=C(OMe) distance 2.479 A) making it the first solid enol of an anhydride. In CDCl3, CD3CN, or C6D6 solution it displays the OH as a broad signal at ca. 15 ppm, suggesting a hydrogen bond with the CO2Me group. NICS calculations indicate that 4 is nonaromatic. With D2O in CDCl3 both the OH and the C5H protons exchange rapidly the H for D. An isomeric anhydride 5a of 5 is formed in equilibrium with 4 in polar solvents. In solution, anhydride(s)/enol equilibria are rapidly established with Kenol of 6.40 (C6D6, 298 K), 0.52 (CD3CN, 298 K), 9.8 (CDCl3, 298 K), 22.8 (CDCl3, 240 K), and decreasing Kenol in CDCl3:CD3CN mixtures with the increase in percent of CD3CN. The percentage of the rearranged anhydride in CDCl3:(CD3)2CO increases with the increased percent of (CD3)2CO. In DMSO-d6 and DMF-d7 the observed species are mainly the conjugated base 4- and 5a. Deuterium effects on the delta(13C) values were determined. An analogous C2-OH enol of anhydride 15 substituted by 3-CO2Me and 4-OCO2Me groups was prepared. Its structure was confirmed by X-ray crystallography (CO bond length 1.298 A, O...O distance 2.513 A); delta(OH) = 12.04-13.22 ppm in CDCl3, THF-d8, and CD3CN, and Kenol = > or = 100, 7.7, and 3.4 respectively. In DMSO-d6 enol 15 ionizes to its conjugate base. Substantial upfield shifts of the apparent delta("OH") proton on diluting the enol solutions are ascribed to the interaction of the H+ formed with the traces of water in the solvent to give H3O+, which gives the alleged "OH proton" signal.     

Different types of hydrogen bonds in 2-substituted pyrroles and 1-vinyl pyrroles as monitored by (1)H, (13)C and (15)N NMR spectroscopy and ab initio calculations

According to the (1)H, (13)C and (15)N NMR spectroscopic data and ab initio calculations, the strong N--H...O intramolecular hydrogen bond in the Z-isomers of 2-(2-acylethenyl)pyrroles causes the decrease in the absolute size of the (1)J(N,H) coupling constant by 2 Hz in CDCl(3) and by 4.5 Hz in DMSO-d(6), the deshielding of the proton and nitrogen by 5-6 and 15 ppm, respectively, and the lengthening of the N--H link by 0.025 A. The N--H...N intramolecular hydrogen bond in the 2(2'-pyridyl)pyrrole leads to the increase of the (1)J(N,H) coupling constant by 3 Hz, the deshielding of the proton by 1.5 ppm and the lengthening of the N--H link by 0.004 A. The C--H...N intramolecular hydrogen bond in the 1-vinyl-2-(2'-pyridyl)-pyrrole results in the increase of the (1)J(C,H) coupling constant by 5 Hz, the deshielding of the proton by 1 ppm and the shortening of the C--H link by 0.003 A. Different behavior of the coupling constants and length of the covalent links under the hydrogen bond influence originate from the different nature of the hydrogen bonding (predominantly covalent or electrostatic), which depends in turn on the geometry of the hydrogen bridge. The Fermi-contact mechanism only is responsible for the increase of the coupling constant in the case of the predominantly electrostatic hydrogen bonding, whereas both Fermi-contact and paramagnetic spin-orbital mechanisms bring about the decrease of coupling constant in the case of the predominantly covalent hydrogen bonding.     

Prediction of a phase transition to a hydrogen bond ordered form of ice VI

Ice VI is a hydrogen bond disordered crystal over its known region of stability. In this work, we predict that ice VI will transform into a hydrogen bond ordered phase near 108 K, and have identified the likely low-temperature phase as ferroelectric (space group Cc) with an antiferroelectric structure (space group P2(1)2(1)2(1)) close by in energy. Electronic density functional theory calculations provide input to our calculations, which are extended to cells large enough for statistical simulations by using graph invariants. A significant decrease in the configurational entropy is predicted as hydrogen bonds exhibit partial order above the transition, provided that the hydrogen bonds can equilibrate on an experimental time scale. Conversely, partial disorder is predicted at temperatures below the transition. Although some evidence for ordering of ice VI has been observed in experiments, a low-temperature proton ordered phase has not been identified experimentally.     

Hydrogen bond in 3-acetyl-4-hydroxycoumarin: X-ray diffraction study and quantum-chemical calculations

The proton transfer and the character of the strong intramolecular O--H...O hydrogen bond (O...O 2.442 ) in 3-acetyl-4-hydroxycoumarin were analyzed based on the results of X-ray diffraction study in the temperature range from 100 to 353 K and quantum-chemical B3LYP/6-31G(d,p) calculations. The barrier to proton transfer along the H-bond line is low (2 kcal mol^–1). However, no proton transfer was observed in the crystal at 100 K. Bader's topological analysis of the electron density distribution both in the crystal and in the isolated molecule demonstrated that the hydrogen bond corresponds to an intermediate type of interatomic interactions (E(r) < 0, ²(r) > 0 at the critical point (3, –1)).     

Dynamics of the intermolecular hydrogen bonds in the polymorphs of paracetamol in relation to crystal packing and conformational transitions: a variable-temperature polarized Raman spectroscopy study

The properties of the intermolecular hydrogen bonds in the monoclinic (Form I) and the orthorhombic (Form II) polymorphs of paracetamol, C(8)H(9)NO(2), have been studied by single crystal polarized Raman spectroscopy (40 to 3700 cm(-1)) in a wide temperature range (5 K < T < 300 K) in relation to the dynamics of methyl-groups of the two forms. A detailed analysis of the temperature dependence of the wavenumbers, bandwidths and integral intensities of the spectral bands has revealed an essential difference between the two polymorphs in the strength and ordering of OH···O and NH···O hydrogen bonds. The compression of intermolecular hydrogen bonds is interrelated with crystal packing and the dynamics of methyl-groups. On structural compression of the orthorhombic polymorph on cooling, a compromise is to be sought between the shortening of OH···O and NH···O bonds, attractive CH···O and repulsive CH···H contacts in the crystal structure. As a result of a steric conflict at temperatures below 100 K, N-H···O hydrogen bonds become significantly disordered, and an extended intramolecular transition from the conformation "staggered" with respect to the C=O bond to the one "staggered" with respect to the NH bond is observed. In most of the studied crystals this transition was only about 60% complete even at 5 K, but in some of the crystals the orientation of all the methyl-groups became staggered with respect to the NH bond at low temperatures. This complete transition was coupled to a sharp shortening of the OH···O and NH···O hydrogen bonds at <100 K, the appearance of new additional positions of the protons in these H-bonds, and a slight strengthening of the C-HO bonds formed by methyl-groups. The same conformational transition has been observed also in the monoclinic polymorph at T < 80 K. The crystal packing in Form I prevents the O-H···O hydrogen bonds from adopting the optimum geometry, and they are significantly disordered at all the temperatures, especially at ≤200 K. The packing of molecules in Form I is also not favourable to form C-H···O hydrogen bonds involving methyl-groups. One can conclude from the comparison of diffraction and spectroscopic data that the higher stability of Form I results not from a larger strength of individual OH···O and NH···O hydrogen bonds, but is a cumulative effect: all the hydrogen bonds together stabilize the structure of the monoclinic polymorph more than that of the orthorhombic polymorph.     

alpha- and beta-FOX-7, polymorphs of a high energy density material, studied by X-ray single crystal and powder investigations in the temperature range from 200 to 423 K

The alpha-beta phase transition in the novel energetic material 1,1-diamino-2,2-dinitroethylene, C2H4N4O4 (FOX-7), has been studied by single-crystal X-ray investigations at five different temperatures over the 200-393 K range. In these investigations, the positions of the hydrogen atoms were experimentally determined without any geometric constraints. In addition, X-ray powder investigations using the Guinier technique have been performed to characterize the beta-phase up to 423 K. The alpha-beta phase transition at 389 K is first order, shows a discontinuous increase of the molar volume and entropy (DeltaV = 1.75 cm3/mol, X-ray investigation; DeltaS = 1.5 cal/K mol, DSC analysis), and can be classified as displacive. The hitherto unknown structure of beta-FOX-7 was solved at 393 K and showed simple structural relations to the alpha-polymorph. The characteristic bonding in wave-shaped layers is now found for beta-FOX-7 (P2(1)2(1)2(1), z = 4, a= 6.9738(7) A, b = 6.635(1) A, c = 11.648(2) A, 393 K), as well as for alpha-FOX-7 (P2(1)/n, z = 4, a = 6.9467(7) A, b = 6.6887(9) A, c = 11.350(1) A, beta = 90.143(13) degrees , 373 K). Interestingly, whereas the intramolecular C-C, C-N, N-O, and N-H bond distances remain nearly unchanged for both polymorphs over the whole temperature range from 200 to 393 K, the two nitro groups deviate strongly from the molecular plane formed by the two carbon and two amino nitrogen atoms. In alpha-FOX-7 at 373 K, the nitro groups are twisted -47 and +6 degrees with respect to the carbon-carbon bond, but in beta-FOX-7 at 393 K, these twist angles are changed to -36 and +20 degrees . Within the layers, the FOX-7 molecules show strong pi-conjugation and extensive intra- and intermolecular hydrogen bonding. In this investigation, we have been able to show that alpha- and beta-FOX-7 build up different nets of intermolecular hydrogen bonds. In alpha-FOX-7, each oxygen atom of the nitro groups is involved in two hydrogen bonds resulting in two intramolecular and six intermolecular hydrogen bonds. But in beta-FOX-7 this coordination changes, and half of the oxygen atoms build up two and the other half build up three hydrogen bonds leading to two intramolecular and eight intermolecular hydrogen bonds. The average intermolecular hydrogen bond distance increases slightly from 2.31 A in alpha-FOX-7 to 2.52 A in beta-FOX-7. The C-NO2 bonds are of particular interest because they are referred to as the detonation trigger. It has been suggested that these bonds could be strengthened by the extensive intermolecular hydrogen bonding within the layers in both polymorphs. Such bond strengthening via cooperative effects was proposed in earlier DFT calculations on FOX-7 and may be one key to understanding its low sensitivity and high activation energy to impact.     

14-helical folding in a cyclobutane-containing beta-tetrapeptide

The efficient synthesis of tetrapeptide 5 containing, in alternation, cyclobutane and beta-alanine residues is described. NMR experiments both at low temperature in CDCl(3) and at 298 K in DMSO-d(6) solutions show the contribution of a strong hydrogen bond in the folded major conformation of 5. Temperature coefficients and diffusion times point out a hydrogen bond involving the NH proton from the cyclobutane residue 1 whereas NOEs manifest the high rigidity of the central fragment of the molecule and are compatible with a 14-membered macrocycle. Theoretical calculations predict a most stable folded conformation corresponding to a 14-helix stabilized by a hydrogen bond between NH(10) in the first residue and OC(25) in the third residue. This structure remains unaltered during the molecular dynamics simulation at 298 K in chloroform. All these results provide evidence for a 14-helical folding and reveal the ability of cis-2-aminocyclobutane carboxylic acid residues to promote folded conformations when incorporated into beta-peptides.     

Proton dynamics and room-temperature ferroelectricity in anilate salts with a proton sponge

Ferroelectricity as well as characteristic proton-transfer dynamics are achieved by combining a 2,3,5,6-tetra(2'-pyridyl)pyrazine (TPPZ) molecule with anilic acids (H2xa). Dielectric measurements revealed phase transitions at T(c) = 334 and 172 K for bromanilate (Hba(-)) and chloranilate (Hca(-)) salts, respectively. The room-temperature ferroelectricity of the (H2-TPPZ)(Hba)2 crystal is evidenced by the slow polarization reversal with modest pyroelectricity. In accord with the observed large deuteration effect, synchrotron X-ray diffraction studies disclosed proton dynamics in an intramolecular N...H(+)...N bond of the H2-TPPZ(2+) dication and in an O-H...O(-) hydrogen-bonded cyclic dimer of the ortho-quinoid Hxa(-) anions. The disordered (Hxa(-))2 dimer in two-fold orientation manifests its double-proton transfer process above T(c), whereas these protons are ordered in the ferroelectric phase. The H2-TPPZ(2+) dication acts as a proton sponge by forming two intramolecular N...H(+)...N hydrogen bridges between the pyridyl units with a very short N...N distance. The dication in the paraelectric state adopts a nonpolar geometry due to the delocalization of the protons over two sites in the respective N...H(+)...N bonds. Below T(c), only one of the two protons gets localized, and the resultant acentric H2-TPPZ(2+) ion generates the dipole moment responsible for the ferroelectricity.     

Geometric isotope effect of various intermolecular and intramolecular C-H...O hydrogen bonds, using the multicomponent molecular orbital method

The geometric isotope effect (GIE) of sp- (acetylene-water), sp(2)- (ethylene-water), and sp(3)- (methane-water) hybridized intermolecular C-H...O and C-D...O hydrogen bonds has been analyzed at the HF/6-31++G level by using the multicomponent molecular orbital method, which directly takes account of the quantum effect of proton/deuteron. In the acetylene-water case, the elongation of C-H length due to the formation of the hydrogen bond is found to be greater than that of C-D. In contrast to sp-type, the contraction of C-H length in methane-water is smaller than that of C-D. After the formation of hydrogen bonds, the C-H length itself in all complexes is longer than C-D and the H...O distance is shorter than D...O, similar to the GIE of conventional hydrogen bonds. Furthermore, the exponent (alpha) value is decreased with the formation of the hydrogen bond, which indicates the stabilization of intermolecular C-H...O hydrogen bonds as well as conventional hydrogen bonds. In addition, the geometric difference induced by the H/D isotope effect of the intramolecular C-H...O hydrogen bond shows the same tendency as that of intermolecular C-H...O. Our study clearly demonstrates that C-H...O hydrogen bonds can be categorized as typical hydrogen bonds from the viewpoint of GIE, irrespective of the hybridizing state of carbon and inter- or intramolecular hydrogen bond.     

Two solid phases of pyrimidin‐1‐ium hydrogen chloranilate monohydrate determined at 225 and 120 K

The crystal structures of two solid phases of the title compound, C₄H₅N₂⁺·C₆HCl₂O₄⁻·H₂O, have been determined at 225 and 120 K. In the high‐temperature phase, stable above 198 K, the transition temperature of which has been determined by ³⁵Cl nuclear quadrupole resonance and differential thermal analysis measurements, the three components are held together by O—H...O, N...H...O, C—H...O and C—H...Cl hydrogen bonds, forming a centrosymmetric 2+2+2 aggregate. In the N...H...O hydrogen bond formed between the pyrimidin‐1‐ium cation and the water molecule, the H atom is disordered over two positions, resulting in two states, viz. pyrimidin‐1‐ium–water and pyrimidine–oxonium. In the low‐temperature phase, the title compound crystallizes in the same monoclinic space group and has a similar molecular packing, but the 2+2+2 aggregate loses the centrosymmetry, resulting in a doubling of the unit cell and two crystallographically independent molecules for each component in the asymmetric unit. The H atom in one N...H...O hydrogen bond between the pyrimidin‐1‐ium cation and the water molecule is disordered, while the H atom in the other hydrogen bond is found to be ordered at the N‐atom site with a long N—H distance [1.10 (3) Å].     

ESIPT-regulated Mechanoresponsive Luminescence Process byIntroducing Intramolecular Hydrogen Bond in NaphthalimideDerivatives#br#

Herein, two compounds, 4-2′-hydroxybenzylidenehydrazinyl-N-butyl-1,8-naphthalimide(BN-1) and 4-benzylidenehydra-zinyl-N-butyl-1,8-naphthalimide(BN-2), were synthesized to explore the hydrogen bonding effect on mechanoresponsive luminescent(MRL). The results showed that compound BN-1 exhibited strong emission in solution and solid-state compared with compound BN-2. After grinding, the emission intensity of compound BN-1 sharply decreased by as much as 15 times with an obvious red-shift from 552 nm to 577 nm. The control compound BN-2, by contrast, did not change so much before and after grinding. Single crystal analysis suggests that BN-1 molecule formed strong intramolecular interaction via ―N=N···H―O hydrogen bond with a distance of 0.2632 nm. An excited-state intramolecular proton transfer(ESIPT) based fluorophore featured this intramolecular hydrogen bond. The intramolecular hydrogen bond as well as other intermolecular interactions can rigidify the molecular conformation of compound BN-1 in solid-state, and thus suppress the nonradiative pathways, resulting in strong emission. These intra- and intermolecular interactions were destroyed by mechanical stimuli, accompanied by molecular conformation change that decreases the luminescence and blocks the ESIPT process. The MRL process was also demonstrated by scanning electron microscopy and powder X-ray diffraction. The molecular stacking mode changed from crystalline to a disordered amorphous state after grinding.     

Charge Transfer in 1,8-Naphthalimide: A Combined Theoretical and Experimental Approach

Photophysics of 1,8-naphthalimide (NAPMD) in different solvents has been delineated in this paper. Theoretically calculated bond distance of N–H and C=O groups rule out any intramolecular proton transfer in the excited state. Concomitant increase in negative charge on O atom compared to N atom and dipole moment hints at possible intramolecular charge transfer. Progressive redshift with polarity of solvents in emission and absorption spectra also confirms the theoretical prediction. Weakening of N–H bond helps hydrogen abstraction and anion formation in water with decay time of 2.54 ns through intermolecular proton transfer. This was corroborated from the ground state photoexcitation of laboratory synthesized anion of NAPMD. Amide hydrolysis in higher pH and excess proton availability at low pH are responsible for anion emission quenching. A possible electron transfer diminishes phosphorescence at 77 K with changing pH.     

Hydrogen-bond dynamics and Fermi resonance in high-pressure methane filled ice

High-pressure, variable temperature infrared spectroscopy and first-principles calculations on the methane filled ice structure (MH-III) at high pressures are used to investigate the vibrational dynamics related to pressure induced modifications in hydrogen bonding. Infrared spectroscopy of isotopically dilute solutions of H(2)O in D(2)O is employed together with first-principles calculations to characterize proton dynamics with the pressure induced shortening of hydrogen bonds. A Fermi resonance is identified and shown to dominate the infrared spectrum in the pressure region between 10 and 30 GPa. Significant differences in the effects of the Fermi resonance observed between 10 and 300 K arise from the double-well potential energy surface of the hydrogen bond and quantum effects associated with the proton dynamics.     

NMR studies of coupled low- and high-barrier hydrogen bonds in pyridoxal-5'-phosphate model systems in polar solution

The 1H and 15N NMR spectra of several 15N-labeled pyridoxal-5'-phosphate model systems have been measured at low temperature in various aprotic and protic solvents of different polarity, i.e., dichloromethane-d2, acetonitrile-d3, tetrahydrofuran-d8, freon mixture CDF3/CDClF2, and methanol. In particular, the 15N-labeled 5'-triisopropyl-silyl ether of N-(pyridoxylidene)-tolylamine (1a), N-(pyridoxylidene)-methylamine (2a), and the Schiff base with 15N-2-methylaspartic acid (3a) and their complexes with proton donors such as triphenylmethanol, phenol, and carboxylic acids of increasing strength were studied. With the use of hydrogen bond correlation techniques, the 1H/15N chemical shift and scalar coupling data could be associated with the geometries of the intermolecular O1H1N1 (pyridine nitrogen) and the intramolecular O2H2N2 (Schiff base) hydrogen bonds. Whereas O1H1N1 is characterized by a series of asymmetric low-barrier hydrogen bonds, the proton in O2H2N2 faces a barrier for proton transfer of medium height. When the substituent on the Schiff base nitrogen is an aromatic ring, the shift of the proton in O1H1N1 from oxygen to nitrogen has little effect on the position of the proton in the O2H2N2 hydrogen bond. By contrast, when the substituent on the Schiff base nitrogen is a methyl group, a proton shift from O to N in O1H1N1 drives the tautomeric equilibrium in O2H2N2 from the neutral O2-H2...N2 to the zwitterionic O2-...H2-N(2+) form. This coupling is lost in aqueous solution where the intramolecular O2H2N2 hydrogen bond is broken by solute-solvent interactions. However, in methanol, which mimics hydrogen bonds to the Schiff base in the enzyme active site, the coupling is preserved. Therefore, the reactivity of Schiff base intermediates in pyridoxal-5'-phosphate enzymes can likely be tuned to the requirements of the reaction being catalyzed by differential protonation of the pyridine nitrogen.     

Vibrational assignment, structure and intramolecular hydrogen bond of 4-methylamino-3-penten-2-one

The molecular structure, intramolecular hydrogen and vibrational frequencies of 4-methylamino-3-penten-2-one were investigated by a series of density functional theoretical (DFT) calculations and ab initio calculation at the post-Hartree-Fock (MP2) level. Fourier transform infrared and Fourier transform Raman spectra of this compound and its deuterated analogue were clearly assigned. The calculated geometrical parameters show a strong intramolecular hydrogen bond with a N...O distance of 2.622-2.670 A. This bond length is about 0.02 A shorter than that in its parent, 4-amino-3- penten-2-one which is in agreement with spectroscopic results. Furthermore, the conformations of methyl groups with respect to the plane of the molecule and with respect to each other were investigated.