Analysis of Proton NMR in Hydrogen Bonds in Terms of Lone‐Pair and Bond Orbital Contributions

Abstract

NMR spectroscopic parameters of the proton involved in hydrogen bonding are studied theoretically. The set of molecules includes systems with internal resonance-assisted hydrogen bonds, internal hydrogen bonds but no resonance stabilization, the acetic acid dimer (AAD), a DNA base pair, and the hydrogen succinate anion (HSA). Ethanol and guanine represent reference molecules without hydrogen bonding. The calculations are based on zero-point vibrationally averaged molecular structures in order to include anharmonicity effects in the NMR parameters. An analysis of the calculated NMR shielding and J-coupling is performed in terms of "chemist's orbitals", that is, localized molecular orbitals (LMOs) representing lone-pairs, atomic cores, and bonds. The LMO analysis associates some of the strong de-shielding of the protons in resonance-assisted hydrogen bonds with delocalization involving the π-backbone. Resonance is also shown to be an important factor causing de-shielding of the OH protons for AAD and HSA, but not for the DNA base pair. Nitromalonamide (NMA) and HSA have particularly strong hydrogen bonds exhibiting signs of covalency in the associated J-couplings. The analysis results show how NMR spectroscopic parameters that are characteristic for hydrogen bonded protons are influenced by the geometry and degree of covalency of the hydrogen bond as well as intra- and intermolecular resonance.

Probing O–H Bonding through Proton Detected1H–17O Double Resonance Solid-State NMR Spectroscopy

“…Autschbach and co-workers have previously applied DFT to calculate 1 J OH in a variety of hydrogen bonded carboxylic acids. 64 They predicted that 1 J OH ranged from 42 Hz to 99 Hz and showed that 1 J OH was correlated to Wiberg bond indices (WBI) which quantify the bond order. 64 Scalar couplings can be measured with a J-resolved 46,[65][66] block at the beginning of the D-RINEPT sequence ( Figure 3C).…”

NMR 1H-Shielding Constants of Hydrogen-Bond Donor Reflect Manifestation of the Pauli Principle

“…29 Other theoretical studies showed that 1 H shielding is determined by σ-type orbital contribution, although in the case of the H-bonded proton, it was shown that 1 H-shielding contributions due to bonds and lone pairs of the acceptor atom are negligible. 26,30−32 Furthermore, hydrogen bonds have a partial covalent character: The unoccupied σ* N–H orbital accepts electronic density from the lone pair of the opposite nitrogen or oxygen atom, which would actually lead to an accumulation of electronic density on the hydrogen atom. 33 In this work, we aim to clarify the electronic contributions in hydrogen bonds that give rise to the observed trend in 1 H-shielding values.…”

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

“…[32][33][34][35][36][37][38][39] Owing to RAHB systems in many organic and coordination compounds, the hydrogen bond is conjugated with a p-electronic system,a nd the 1 HNMR resonance of the involved proton( > 12 ppm) is shifteds trongly downfield relative to that for ar egular hydrogen bond. Nevertheless, in originals ources, these roles of RAHB have often been disregarded,a nd some authors have even criticizedt he concept of RAHB in af ew cases.…”

“…When doubleb onds are not bridged only by as ingle bond (e.g.,i n6 -, 7-, or 8-membered H-bonded rings), thush ampering the conjugation,t hey are not involved in RAHB systems, [31][32][33][34][35][36][37][38][39] and are out of the scope of this Review.E xamples include dimers of aldehydes, of carboxylic acids and of pyridin-2-ol, as well as fulvenes (Scheme 2a-d), [31][32][33][34][35][36][37] and contrast, for example, with the case of b-diketones [24,[40][41][42] and( Z)-6-hydroxy-5-[(1methyl-2-oxoindolin-3-ylidene)methyl]pyrimidine-2,4(1 H,3H)dione [43] (Scheme 2e), in which resonance stabilization has ac onsiderable effect. [31][32][33][34][35][36][37][38][39] As mentioned above,i nR AHB, the hydrogen-bond donor and acceptora toms are connected through p-conjugated double bonds.…”