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.     

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.     

Systematic ab initio study of 15N-15N and 15N-1H spin-spin coupling constants across N-H+-N hydrogen bonds: predicting N-N and N-H coupling constants and relating them to hydrogen bond type

A systematic ab initio EOM-CCSD study of 15N-15N and 15N-1H spin-spin coupling constants has been carried out for a series of complexes formed from 11 nitrogen bases with experimentally measured proton affinities. When these complexes are arranged in order of increasing proton affinity of the proton-acceptor base and, for each proton acceptor, increasing order of proton affinity of the protonated N-H donor, trends in distances and signs of coupling constants are evident that are indicative of the nature of the hydrogen bond. All two-bond spin-spin coupling constants (2hJ(N-N)) are positive and decrease as the N-N distance increases. All one-bond N-H coupling constants (1J(N-H)) are negative (1K(N-H) are positive). 1J(N-H) is related to the N-H distance and the hybridization of the donor N atom. One-bond H...N coupling constants (1hJ(H-N)) are positive (1hK(H-N) are negative) for traditional hydrogen bonds, but 1hJ(H-N) becomes negative when the hydrogen bond acquires sufficient proton-shared character. The N-N and H...N distances at which 1hJ(H-N) changes sign are approximately 2.71 and 1.62 A, respectively. Predictions are made of the values of 2hJ(N-N) and 1J(N-H), and the signs of 1hJ(H-N), for those complexes that are too large for EOM-CCSD calculations.     

Theoretical study of bifurcated hydrogen bonding effects on the 1J(N,H), 1hJ(N,H), 2hJ(N,N) couplings and 1H, 15N shieldings in model pyrroles

According to the density functional theory calculations, the X···H···N (X?N, O) intramolecular bifurcated (three‐centered) hydrogen bond with one hydrogen donor and two hydrogen acceptors causes a significant decrease of the ^1hJ(N,H) and ^2hJ(N,N) coupling constants across the N? H···N hydrogen bond and an increase of the ¹J(N,H) coupling constant across the N? H covalent bond in the 2,5‐disubsituted pyrroles. This occurs due to a weakening of the N? H···N hydrogen bridge resulting in a lengthening of the N···H distance and a decrease of the hydrogen bond angle at the bifurcated hydrogen bond formation. The gauge‐independent atomic orbital calculations of the shielding constants suggest that a weakening of the N? H···N hydrogen bridge in case of the three‐centered hydrogen bond yields a shielding of the bridge proton and deshielding of the acceptor nitrogen atom. The atoms‐in‐molecules analysis shows that an attenuation of the ^1hJ(N,H) and ^2hJ(N,N) couplings in the compounds with bifurcated hydrogen bond is connected with a decrease of the electron density ρ_H···N at the hydrogen bond critical point and Laplacian of this electron density ?²ρ_H···N. The natural bond orbital analysis suggests that the additional N? H···X interaction partly inhibits the charge transfer from the nitrogen lone pair to the σ*_{N? H} antibonding orbital across hydrogen bond weakening of the ^1hJ(N,H) and ^2hJ(N,N) trans‐hydrogen bond couplings through Fermi‐contact mechanism. An increase of the nitrogen s‐character percentage of the N? H bond in consequence of the bifurcated hydrogen bonding leads to an increase of the ¹J(N,H) coupling constant across the N? H covalent bond and deshielding of the hydrogen donor nitrogen atom. Copyright © 2010 John Wiley & Sons, Ltd.     

Tautomerism in the solid state and in solution of a series of 6-aminofulvene-1-aldimines

To study systems able to sustain intramolecular proton-transfer, we have prepared a series of six aminofulvene aldimines including several labeled with (15)N and (2)H. These compounds show coupling constants through the hydrogen bond, (1h)J((15)N- (1)H) and (2h)J((15)N-(15)N). The position of the tautomeric equilibria, i.e., on what nitrogen atom is the proton, was determined in the solid state and in solution. The crystal structure of N[[5-[(phenylamino)methylene]-1,3-cyclopentadien-1-yl]methylene]pyrrole-1-amine (3) has been determined by X-ray analysis. In solution, both N-H and C-H tautomers were observed and their structures assigned by NMR spectroscopy. Particularly useful is the value of the (1)J((15)N-(1)H) coupling constant.     

Ab initio study of ternary complexes X:(HCNH)(+):Z with X, Z = NCH, CNH, FH, ClH, and FCl: diminutive cooperative effects on structures, binding energies, and spin-spin coupling constants across hydrogen bonds

Ab initio calculations have been performed on a series of complexes in which (HCNH)(+) is the proton donor and CNH, NCH, FH, ClH, and FCl (molecules X and Z) are the proton acceptors in binary complexes X:HCNH(+) and HCNH(+):Z, and ternary complexes X:HCNH(+):Z. These complexes are stabilized by C-H(+)···A and N-H(+)···A hydrogen bonds, where A is the electron-pair donor atom of molecules X and Z. Binding energies of the ternary complexes are less than the sum of the binding energies of the corresponding binary complexes. In general, as the binding energy of the binary complex increases, the diminutive cooperative effect increases. The structures of these complexes, data from the AIM analyses, and coupling constants (1)J(N-H), (1h)J(H-A), and (2h)J(N-A) for the N-H(+)···A hydrogen bonds, and (1)J(C-H), (1h)J(H-A), and (2h)J(C-A) for the C-H(+)···A hydrogen bonds provide convincing evidence of diminutive cooperative effects in these ternary complexes. In particular, the symmetric N···H(+)···N hydrogen bond in HCNH(+):NCH looses proton-shared character in the ternary complexes X:HCNH(+):NCH, while the proton-shared character of the C···H(+)···C hydrogen bond in HNC:HCNH(+) decreases in the ternary complexes HNC:HCNH(+):Z and eventually becomes a traditional hydrogen bond as the strength of the HCNH(+)···Z interaction increases.     

Ab initio EOM-CCSD spin-spin coupling constants for hydrogen-bonded formamide complexes: bridging complexes with NH3, (NH3)2, H2O, (H2O)2, FH, and (FH)2

EOM-CCSD spin-spin coupling constants across hydrogen bonds have been computed for complexes in which NH3, H2O, and FH molecules and their hydrogen-bonded dimers form bridging complexes in the amide region of formamide. The formamide one-bond N-H coupling constant [(1)J(N-H)] across N-H...X hydrogen bonds increases in absolute value upon complexation. The signs of the one-bond coupling constants (1h)J(H-X) indicate that these complexes are stabilized by traditional hydrogen bonds. The two-bond coupling constants for hydrogen bonds with N-H as the donor [(2h)J(N-X)] and the carbonyl oxygen as the acceptor [(2h)J(X-O)] increase in absolute value in the formamide/dimer relative to the corresponding formamide/monomer complex as the hydrogen bonds acquire increased proton-shared character. The largest changes in coupling constants are found for complexes of formamide with FH and (FH)2, suggesting that bridging FH monomers and dimers in particular could be useful NMR spectroscopic probes of amide hydrogen bonding.     

Ab initio study of complexes with two cations as N-H donors to F-: structures and spin-spin coupling constants across N-H-F hydrogen bonds

A systematic ab initio study has been carried out to determine the MP2/6-31+G(d,p) structures and EOM-CCSD coupling constants across N-H-F-H-N hydrogen bonds for a series of complexes F(H(3)NH)(2)(+), F(HNNH(2))(2)(+), F(H(2)CNH(2))(2)(+), F(HCNH)(2)(+), and F(FCNH)(2)(+). These complexes have hydrogen bonds with two equivalent N-H donors to F(-). As the basicity of the nitrogen donor decreases, the N-H distance increases and the N-H-F-H-N arrangement changes from linear to bent. As these changes occur and the hydrogen bonds between the ion pairs acquire increased proton-shared character, (2h)J(F)(-)(N) increases in absolute value and (1h)J(H)(-)(F) changes sign. F(H(3)NH)(2)(+) complexes were also optimized as a function of the N-H distance. As this distance increases and the N-H...F hydrogen bonds change from ion-pair to proton-shared to traditional F-H...N hydrogen bonds, (2h)J(F)(-)(N) initially increases and then decreases in absolute value, (1)J(N)(-)(H) decreases in absolute value, and (1h)J(H)(-)(F) changes sign. The signs and magnitudes of these coupling constants computed for F(H(3)NH)(2)(+) at short N-H distances are in agreement with the experimental signs and magnitudes determined for the F(collidineH)(2)(+) complex in solution. However, even when the N-H and F-H distances are taken from the optimized structure of F(collidineH)(2)(+), (2h)J(F)(-)(N) and (1h)J(H)(-)(F) are still too large relative to experiment. When the distances extracted from the experimental NMR data are used, there is excellent agreement between computed and experimental coupling constants. This suggests that the N-H-F hydrogen bonds in the isolated gas-phase F(collidineH)(2)(+) complex have too much proton-shared character relative to those that exist in solution.     

Scalar Coupling between the 15N Centres in Methylated 1,8‐Diaminonaphthalenes and 1,6‐Diazacyclodecane: To What Extent is 2HJNN a Reliable Indicator of N?N Distance?

The scalar couplings between hydrogen bonded nitrogen centres ((2H)J(NN)) in the free-base and protonated forms of the complete series of [(15)N(2)]-N-methylated 1,8-diamino naphthalenes in [D(7)]DMF solution have been determined, either directly (15N[1H] NMR), or, indirectly (13C[1H] NMR and simulation of the X part of the ABX spectrum (X=13C, A,B=15N)). Additionally, the (2H)J(NN) value in the HBF(4) salt of [(15)N(2)]-1,6-dimethyl-1,6-diazacyclodecane was determined, indirectly by 13C[(1H] NMR spectroscopy. As confirmed by DFT calculations and by reference to CSD, the rigid nature of the naphthalene scaffold results in rather low deviations in N,N distance or H-N,N angle within each series, apart from the free base of the permethylated compound (proton sponge) where the naphthalene ring is severely distorted to relieve strain. Despite such restrictions, the (2H)J(NN) values increase smoothly from 1.5 to 8.5 Hz in the protonated series as the degree of methylation increases. The effect in the free-base forms is much less pronounced (2.9 to 3.7 Hz) with no scalar N,N coupling detected in the permethylated compound (proton sponge) due to the lack of hydrogen bond between the N,N centres. Neither the pK(a) nor the N-N distance in the protonated forms correlates with (2H)J(NN). However, the sum of the (13)C NMR shifts of the naphthalene ring C(1,8) carbons which are attached directly to the nitrogen centres correlates linearly with (2H)J(NN) and with the degree of methylation. The gas-phase computed (2H)J(NN) is almost constant throughout the homologous series, and close to the experimental value for the tetramethylated ion. However, the computed coupling constant is attenuated in structures involving microsolvation of each N-H unit, and the trend then matches experiment. These experimental and computational observations suggest that Fermi contact between the two N centres is decreased upon formation of strong charge-dispersing intermolecular hydrogen bonds of the free N-H groups with the solvent.     

Synthesis and in solution and solid state structural study of intramolecular Pt...H interactions in pentafluorophenyl platinum(II) complexes containing the ligands triazene, formamidine, and 2-aminopyridine

The preparation of the [NBu4][Pt(C6F5)3L] complexes (L=triazene, formamidine, 2-aminopyridine,) have been carried out. These ligands contain a hydrogen atom, with more or less acidic character, in a position suitable for establishing an intramolecular hydrogen bonding interaction with the metal center. This interaction has been detected in solution for; its 1H NMR spectrum shows that the resonance assignable to this hydrogen has platinum satellites. For, this coupling is not observed, and the interaction, if it exists, has to be weaker because of the less acidic character of the hydrogen atom. The 2-aminopyridine ligand is more flexible than the triazene or formamidine, and also in this case, no evidence of the interaction in solution is obtained. Nevertheless, if another potential proton acceptor is present, such as ClO4- in [NBu4]2[Pt(C6F5)3(C5H6N2)](ClO4), a conventional N-H...O-Cl hydrogen bond is formed. The crystal structures of complexes have been determined by X-ray diffraction.     

Ab initio study of the influence of trimer formation on one- and two-bond spin-spin coupling constants across an X-H-Y hydrogen bond: AH:XH:YH3 complexes for A, X = 19F, 35Cl and Y = 15N, 31P

Ab initio equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) calculations have been carried out to investigate the effect of a third polar near-neighbor on one-bond ((1)J(X)(-)(H) and (1h)J(H)(-)(Y)) and two-bond ((2h)J(X)(-)(Y)) spin-spin coupling constants in AH:XH:YH(3) complexes, where A and X are (19)F and (35)Cl and Y is either (15)N or (31)P. The changes in both one- and two-bond spin-spin coupling constants upon trimer formation indicate that the presence of a third molecule promotes proton transfer across the X-H-Y hydrogen bond. The proton-shared character of the X-H-Y hydrogen bond increases in the order XH:YH(3) < ClH:XH:YH(3) < FH:XH:YH(3). This order is also the order of decreasing shielding of the hydrogen-bonded proton and decreasing X-Y distance, and is consistent with the greater hydrogen-bonding ability of HF compared to HCl as the third molecule. For all complexes, the reduced X-H and X-Y spin-spin coupling constants ((1)K(X)(-)(H) and (2h)K(X)(-)(Y)) are positive, consistent with previous studies of complexes in which X and Y are second-period elements in hydrogen-bonded dimers. (1h)K(H)(-)(Y) is, as expected, negative in these complexes which have traditional hydrogen bonds, except for ClH:FH:NH(3) and FH:FH:NH(3). In these two complexes, the F-H-N hydrogen bond has sufficient proton-shared character to induce a change of sign in (1h)K(H)(-)(Y). The effects of trimer formation on spin-spin coupling constants are markedly greater in complexes in which NH(3) rather than PH(3) is the proton acceptor.     

Investigation of C–HX (X=N, O, S) intramolecular hydrogen bond in 1-vinyl-2-(2′-heteroaryl)pyrroles by ab initio calculations

The C–HX (X=N, O, S) intramolecular hydrogen bond between the α-hydrogen of the vinyl group and the corresponding heteroatom in the series of 1-vinyl-2-(2′-heteroaryl)pyrroles was examined by ab initio calculations at the B3LYP/6-311(d,p) level. It was shown that the C–HN hydrogen bond is stronger than the C–HO hydrogen bond and the latter is, in turn, stronger than the C–HS hydrogen bond. This conclusion is supported by calculations of ¹H NMR chemical shieldings.     

The empirical correlation between hydrogen bonding strength and excited-state intramolecular proton transfer in 2-pyridyl pyrazoles

A series of 2-pyridyl pyrazoles 1a and 1-5 with various functional groups attached to either pyrazole or pyridyl moieties have been strategically designed and synthesized in an aim to probe the hydrogen bonding strength in the ground state versus dynamics of excited-state intramolecular proton transfer (ESIPT) reaction. The title compounds all possess a five-membered-ring (pyrazole)N-H···N(pyridine) intramolecular hydrogen bond, in which both the N-H bond and the electron density distribution of the pyridyl nitrogen lone-pair electrons are rather directional, so that the hydrogen bonding strength is relatively weak, which is sensitive to the perturbation of subtle chemical substitution and consequently reflected from the associated ESIPT dynamics. Various approaches such as (1)H NMR (N-H proton) to probe the hydrogen bonding strength and absorption titration to assess the acidity-basicity property were made for all the title analogues. The results, together with supplementary support provided by a computational approach, affirm that the increase of acidity (basicity) on the hydrogen bonding donor (acceptor) sites leads to an increase of hydrogen-bonding strength among the title 2-pyridyl pyrazoles. Luminescence results and the associated ESIPT dynamics further reveal an empirical correlation in that the increase of the hydrogen bonding strength leads to an increase of the rate of ESIPT for the title 2-pyridyl pyrazoles, demonstrating an interesting relationship among N-H acidity, hydrogen bonding strength, and the associated ESIPT rate.     

Electron bridging dihydrogen bond in the imidazole-contained anion derivatives

The large contact distance of electron bridging dihydrogen bond (EBDB), which is over 2.4 A, is the most prominent characteristic for the imidazole-contained anion derivatives. The elongation of N-H bond and the shortening of H...H distance can be observed upon hydration and hydrogenation. Transformation from EBDB to dissociative H2 is convenient upon sequential hydrogenation. The H...H distance decreases with the enhancement of the electronegativity of the heavy atom which contacts directly with one of these two hydrogen atoms. NMR shielding of the bonding N varies significantly upon hydration and hydrogenation. The spin-spin coupling constants, 1J(H-H), is dominated predominantly by the paramagnetic spin-orbit and diamagnetic spin-orbit contributions instead of the Fermi-contact term. Enhancement of electronegativity of the heavy atom leads to the increase of 1J(H-H) coupling constants. The stabilization is enhanced upon hydration predominantly for the formation of O-H...N H bond, while it is reversed upon hydrogenation for the cleavage of big pi bond, Pi5(6). Enhancement of the stability is demonstrated by the increase of stabilization energy and vertical electron detachment energy with the electronegativity of the heavy atom. The dominant contributions for the formation of such electron bridging dihydrogen bond are the high polarity of each fragment, large electron density between two fragments, and strong bonding interaction of the bridging electron with H(N) atoms. The H...H interaction can be formed by X-Hdelta+ and Hdelta- -Y polar molecules in Hdelta+...Hdelta- and Hdelta+...e...Hdelta+ of two forms.     

Low-temperature NMR studies of the structure and dynamics of a novel series of acid-base complexes of HF with collidine exhibiting scalar couplings across hydrogen bonds

The low-temperature (1)H, (19)F, and (15)N NMR spectra of mixtures of collidine-(15)N (2,4,6-trimethylpyridine-(15)N, Col) with HF have been measured using CDF(3)/CDF(2)Cl as a solvent in the temperature range 94-170 K. Below 140 K, the slow proton and hydrogen bond exchange regime is reached where four hydrogen-bonded complexes between collidine and HF with the compositions 1:1, 2:3, 1:2, and 1:3 could be observed and assigned. For these complexes, chemical shifts and scalar coupling constants across the (19)F(1)H(19)F and (19)F(1)H(15)N hydrogen bridges have been measured which allowed us to determine the chemical composition of the complexes. The simplest complex, collidine hydrofluoride ColHF, is characterized at low temperatures by a structure intermediate between a molecular and a zwitterionic complex. Its NMR parameters depend strongly on temperature and the polarity of the solvent. The 2:3 complex [ColHFHCol](+)[FHF](-) is a contact ion pair. Collidinium hydrogen difluoride [ColH](+)[FHF](-) is an ionic salt exhibiting a strong hydrogen bond between collidinium and the [FHF](-) anion. In this complex, the anion [FHF](-) is subject to a fast reorientation rendering both fluorine atoms equivalent in the NMR time scale with an activation energy of about 5 kcal mol(-)(1) for the reorientation. Finally, collidinium dihydrogen trifluoride [ColH](+)[F(HF)(2)](-) is an ionic pair exhibiting one FHN and two FHF hydrogen bonds. Together with the [F(HF)(n)()](-) clusters studied previously (Shenderovich et al., Phys. Chem. Chem. Phys. 2002, 4, 5488), the new complexes represent an interesting model system where the evolution of scalar couplings between the heavy atoms and between the proton and the heavy atoms of hydrogen bonds can be studied. As in the related FHF case, we observe also for the FHN case a sign change of the coupling constant (1)J(FH) when the F.H distance is increased and the proton shifted to nitrogen. When the sign change occurs, that is, (1)J(FH) = 0, the heavy atom coupling constant (2)J(FN) remains very large, of the order of 95 Hz. Using the valence bond order model and hydrogen bond correlations, we describe the dependence of the hydrogen bond coupling constants, of hydrogen bond chemical shifts, and of some H/D isotope effects on the latter as a function of the hydrogen bond geometries.     

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.     

A DFT study of the interresidue dependencies of scalar J-coupling and magnetic shielding in the hydrogen-bonding regions of a DNA triplex.

Scalar coupling constants and magnetic shieldings in the imino hydrogen-bonding region of Hoogsteen-Watson-Crick T.A-T and C(+).G-C triplets have been calculated as a function of the distance between proton donor and acceptor nitrogen atoms. The Fermi contact contributions to (h2)J((15)N-H...(15)N), (1)J((15)N-(1)H), and (h1)J((1)H...(15)N) were computed using density functional theory/finite perturbation theory (DFT/FPT) methods for the full base triplets at the unrestricted B3PW91/6-311G level. Chemical shifts delta((1)H) and delta((15)N) were obtained at the same level using the gauge including atomic orbital (GIAO) method for magnetic shielding. All three scalar couplings and all three chemical shifts are strongly interrelated and exhibit monotonic changes with base pair separation. These correlations are in conformity with experimental data for a 32-nucleotide DNA triplex. The results suggest that both chemical shifts and coupling constants can be used to gain information on H-bond donor-acceptor distances in nucleic acids. In addition to the DFT/FPT calculations, a simple three-orbital model of the N-H...H bond and a sum-over-states analysis is presented. This model reproduces the basic features of the H-bond coupling effect. In accordance with this model and the DFT calculations, a positive sign for the (h2)J(NN) coupling is determined from an E.COSY experiment.     

A nitrogen-15 NMR study of hydrogen bonding in 1-alkyl-4-imino-1,4-dihydro-3-quinolinecarboxylic acids and related compounds

The title compounds contain groups (amine, amide, imine, carboxylic acid) that are capable of forming intramolecular hydrogen bonds involving a six-membered ring. In compounds where the two interacting functional groups are imine and carboxylic acid, the imine is protonated to give a zwitterion; where the two groups are imine and amide, the amide remains intact and forms a hydrogen bond to the imine nitrogen. The former is confirmed by the iminium 15N signal, which shows the coupling of 1J(15N,1H) -85 to -86.8 Hz and 3J(1H,1H) 3.7-4.2 Hz between the iminium proton and the methine proton of a cyclopropyl substituent on the iminium nitrogen. Hydrogen bonding of the amide is confirmed by its high 1H chemical shift and by coupling of the amide hydrogen to (amide) nitrogen [(1J(15N,1H) -84.7 to -90.7 Hz)] and to ortho carbons of a phenyl substituent. Data obtained from N,N-dimethylanthranilic acid show 15N-1H coupling of (-)8.2 Hz at 223 K (increasing to (-)5.3 Hz at 243 K) consistent with the presence of a N... H-O hydrogen bond.     

GIAO,DFT, AIM and NBO analysis of the N?H···O intramolecular hydrogen‐bond influence on the 1J(N,H) coupling constant in push–pull diaminoenones

In the series of diaminoenones, large high‐frequency shifts of the ¹H NMR of the N? H group in the cis‐position relative to the carbonyl group suggests strong N? H···O intramolecular hydrogen bonding comprising a six‐membered chelate ring. The N? H···O hydrogen bond causes an increase of the ¹J(N,H) coupling constant by 2–4 Hz and high‐frequency shift of the ¹⁵N signal by 9–10 ppm despite of the lengthening of the relevant N? H bond. These experimental trends are substantiated by gauge‐independent atomic orbital and density functional theory calculations of the shielding and coupling constants in the 3,3‐bis(isopropylamino)‐1‐(aryl)prop‐2‐en‐1‐one (12) for conformations with the Z‐ and E‐orientations of the carbonyl group relative to the N? H group. The effects of the N? H···O hydrogen‐bond on the NMR parameters are analyzed with the atoms‐in‐molecules (AIM) and natural bond orbital (NBO) methods. The AIM method indicates a weakening of the N? H···O hydrogen bond as compared with that of 1,1‐di(pyrrol‐2‐yl)‐2‐formylethene (13) where N? H···O hydrogen bridge establishes a seven‐membered chelate ring, and the corresponding ¹J(N,H) coupling constant decreases. The NBO method reveals that the LP(O) →σ*_{N? H} hyperconjugative interaction is weakened on going from the six‐membered chelate ring to the seven‐membered one due to a more bent hydrogen bond in the former case. A dominating effect of the N? H bond rehybridization, owing to an electrostatic term in the hydrogen bonding, seems to provide an increase of the ¹J(N,H) value as a consequence of the N? H···O hydrogen bonding in the studied diaminoenones. Copyright © 2010 John Wiley & Sons, Ltd.     

Calculations of the contribution of ring currents to the chemical shielding anisotropy

Ring currents can exert a large effect upon the chemical shielding of NMR resonances. The analytical expression developed by Waugh and Fessenden (Waugh, J. S.; Fessenden, R. W. J. Am. Chem. Soc. 1957, 79, 846) and Johnson and Bovey (Johnson, C. E.; Bovey, F. A. J. Chem. Phys. 1957, 29, 1012) only quantifies the contribution of ring currents to the isotropic component of the shielding tensor. In the work described here an additional analytical expression is developed so that the contribution of ring currents to the full shielding tensor can be calculated, allowing an estimate of the influence of ring currents upon the chemical shielding anisotropy (CSA, Deltasigma). To test that this pair of analytical expressions can provide a reasonable estimate of the contribution of ring currents to the full shielding tensor a series of density functional calculations (DFT) were carried out. A shielding tensor in a model compound was calculated in two distinct ways. For the first series, DFT shielding calculations of the model compound were carried out in the presence of a benzene ring. For the second series a ring current contribution to the shielding tensor was calculated via the new expressions, and this was added to the result of a DFT shielding calculation which used in place of benzene the nonaromatic analogue 1,3 cyclohexadiene. The two series of results proved to be in excellent agreement. The pair of analytical expressions are used to calculate ring current contributions to the CSA (Deltasigma) of 1H(N) backbone amide resonances in a structure of the second type 2 module from the protein fibronectin. Significant CSA variations are predicted in particular for the 1H(N) of G42 which is most likely involved in a N-H...tpi aromatic hydrogen bond.