NH Proton Donors Research Articles

Intramolecular Natural Energy Decomposition Analysis: Applications to the Rational Design of Foldamers.

We describe an intramolecular version of the natural energy decomposition analysis (NEDA), with the aim of evaluating interactions between molecular fragments across covalent bonds. The electronic energy in intramolecular natural energy decomposition analysis (INEDA) is divided into electrical, core, and charge transfer components. The INEDA method describes the fragments using the nonfragmented electronic density, and, therefore, there are no limitations in how to choose the boundary orbital. We used INEDA to evaluate the interaction energies that give origin to barriers of rotation around Camide Caromatic (Cam Car ) and Namide Caromtaic (Nam Car ) bonds in arylamide-foldamer building blocks. We found that differences of barrier height between models with different ortho-aryl substituents stem from charge transfer and core interactions. In three-center hydrogen-bond (H-bond) models with an NH proton donor H-bound to two electronegative ortho-aryl substituents, the interaction energy of the three-center system is larger than in either of the two-center H-bond subsystem alone, indicating an increase of overall rigidity. The combination of INEDA and NEDA allows the evaluation of intermolecular and intramolecular interactions using a consistent theoretical framework. © 2018 Wiley Periodicals, Inc.

Angular dependence of hydrogen bond energy in neutral and charged systems containing CH and NH proton donors

The effects of angular distortions on the H-bond energy are computed in both neutral and ionic complexes. F3CH, NCH, and HNCH+ are taken as CH donors and HCNH, HCNH+, and NH4+ are NH donors. Ionic complexes are more strongly bound and suffer a greater loss of H-bond energy upon angular distortion. However, when bending force constants k are normalized against intrinsic H-bond strength Eb, the k/Eb ratios are similar, only slightly larger for NH than for CH donors, and with only small perturbations caused by overall charge. The source of destabilization arising from angular deformation is traced to exchange repulsion.

Competition between non-classical and classical hydrogen bonded sites in [BH 3CN] −: Spectral, energetic, structural and electronic features

Interaction of the salt (Ph 3P N PPh 3)BH 3CN with the various OH and NH proton donors in low polar media was studied by variable temperature (200–290 K) IR spectroscopy and theoretically by DFT calculations. The formation of two types of complexes containing non-classical dihydrogen bond to the hydride hydrogen (DHB) and classical hydrogen bond (HB) to nitrogen lone pair was shown in solution. The 1:1 complexes of both types (XH⋯H and XH⋯N) coexist in the presence of equimolar amount of proton donor. The addition of excess XH-acid leads to the increase of the classical HB content and appearance of the 1:2 complexes, where two basic sites work simultaneously. The structure, spectral characteristics, energy and electron redistribution were studied by DFT (B3LYP) method. The comparison DHB parameters of [BH 3CN] − with those of the unsubstituted analogue [BH 4] − allowed analyzing the electronic effects of the CN group on the basic properties of boron hydride moiety. The electronic influence of the BH 3 group on CN −⋯HX hydrogen bond was also established by comparison with the corresponding classical HB to the CN − anion.

2-Aryl-8-chloro-1,2,4-triazolo[1,5- a]quinoxalin-4-amines as highly potent A 1 and A 3 adenosine receptor antagonists

Some 2-aryl-8-chloro-1,2,4-triazolo[1,5- a]quinoxaline derivatives 2– 18, obtained by introducing different substituents on either the 4-amino moiety (acyl or carbamoyl groups) or the 2-phenyl ring (4-OCH 3) of previously reported 8-chloro-2-phenyl-1,2,4-triazolo[1,5- a]quinoxalin-4-amine ( 1), have been synthesized and tested in radioligand binding assays at bovine A 1 and A 2A and at cloned human A 1 and A 3 adenosine receptors. The rationally designed 8-chloro-2-(4-methoxy-phenyl)-1,2,4-triazolo[1,5- a]quinoxalin-4-acetylamine ( 14) can be considered one of the most potent and hA 3 versus hA 1 selective AR antagonists reported till now. The structure–activity relationships of compounds 2– 18 are in agreement with those of previously reported 2-aryl-1,2,4-triazolo[4,3- a]quinoxalines (series A) and 2-arylpyrazolo[3,4- c]quinolines (series B), thus suggesting a similar AR binding mode. In fact, the importance for the A 3 receptor–ligand interaction of both a strong acidic NH proton donor and a C O proton acceptor at position-4, able to engage hydrogen-bonding interactions with specific sites on the A 3 AR, has been confirmed. Using our recently published hA 3 receptor model, to better elucidate our experimental results, we decided to theoretically depict the putative TM binding motif of the herein reported 1,2,4-triazolo[1,5- a]quinoxaline derivatives on human A 3 receptor. Structure–activity relationships have been explained analyzing the three-dimensional structure of the antagonist-receptor models obtained by molecular docking simulation.

Computational Study of Hydrogen-Bonded Complexes between the Most Stable Tautomers of Glycine and Uracil

A total of 23 hydrogen bonded complexes between the lowest energy tautomers of uracil and glycine have been characterized at the density functional level of theory with a hybrid B3LYP exchange-correlation functional and 6-31++G** basis sets. The most stable complexes are formed when the carboxylic group of glycine is bound through two hydrogen bonds with a NH proton donor and an O proton acceptor of uracil, and stabilization energies for these cyclic structures span a range of 12.3−15.6 kcal/mol. Interplay between the topological match among proton donor and acceptor sites involved in cyclic structures and their preference to form single hydrogen bonds, measured by the values of proton affinity and deprotonation enthalpy, has been discussed. Upon formation of a uracil−glycine complex, the elongations of proton donor bonds and vibrational red shifts for proton donor stretching modes can reach 0.05 A and 650 cm-1, respectively. These perturbations of proton donor bonds correlate with the magnitude of two-bo...

Topochemical Control of Covalent Bond Formation by Dihydrogen Bonding

This report explores the potential of the unconventional hydrogen bonds between the hydridic hydrogens in X−BH3- (X = H, CN) and traditional OH or NH proton donors (dihydrogen bonds) to serve as preorganizing interactions for the topochemical assembly of covalent materials. Evidence for such topochemical control in the reaction B−H···H−X → B−X + H2 was obtained in studies of the solid-state structures and reactivities of N-[2-(6-aminopyridyl)]acetamidine (NAPA) cyanoborohydride and triethanolamine (TEA) complexes of NaBH4 and NaCNBH3. The X-ray crystal structures of all three new compounds studied exhibit multiple dihydrogen bonds which are significant in defining the packing of the molecules in the solid state. Moreover, this new type of interaction is a powerful tool for crystal engineering; as planned, NAPA H3BCN crystallized in the desired (NAPA H3BCN)2 closed loop coordination. Solid-state decomposition of NaBH4·TEA is topochemical, leading to a trialkoxyborohydride, which is not achievable in solution or melt. In addition to close H−H contacts, the relative acidity/basicity of the proton−hydride pairs make a significant contribution to the solid-state reactivity of dihydrogen-bonded systems, as demonstrated by the contrasting reactivities of the NaBH4·TEA and NaCNBH3·TEA complexes.

Thermodynamic and spectroscopic properties of phosphorus compounds—complex formation between phosphorus compounds and various OH and NH proton donors

Thermodynamic and spectral quantities for the 1:1 complex formation between various OH and NH proton donors and PX electron donors where X is O, S or Se were determined in carbon tetrachloride at four temperatures by a near i.r. spectroscopic method. The wavenumber shifts Δ ν OH/NH of the OH/NH band of proton donors were found to vary in the same direction as the corresponding thermodynamic quantities Δ G 0 and Δ H and consistently with the p K a value of the proton donor. The quantities decrease for the phosphorus compounds in the order: triphenylphosphine oxide > trimethylphosphate triphenylphosphate > triphenylphosphine sulfide μ triphenylphosphine selenide, and for the proton donors in the order 2-naphthol > 1-naphthol > phenol > 2,2,2-trifluoroethanol > 2,2,2-trichloroethanol > 2,2,2-tribromoethanol > carbazole > indole > pyrrole. The results are in agreement with our earlier findings.