Hydrogen-Bond Scalar Couplings as Covalency-Sensitive NMR Fingerprints of Amorphous Ice.

Through-hydrogen-bond scalar couplings are attractive NMR observables because they connect high-precision spectroscopy with local hydrogen-bond structure. It is less clear whether they can also report hydrogen-bond covalency in amorphous ice and other frozen or heterogeneous aqueous environments. Here, we combine ab initio molecular dynamics configurations of water, density functional response calculations of indirect nuclear spin-spin couplings, and absolutely localized molecular orbital (ALMO) energy decomposition analysis. Benchmark calculations against SOPPA(CCSD) water-dimer references validate BLYP/pcJ-1 for the through-hydrogen-bond 1hJO-H coupling. The coupling is dominated by the Fermi contact term and therefore follows an approximately exponential distance dependence, but ensemble and vibrational averaging prevent a transferable one-dimensional distance ruler. Extending earlier NMR/ALMO work on liquid water, 1hJO-H correlates with ALMO charge-transfer stabilization and charge-transfer amount. Thus, 1hJO-H is an experimentally accessible, covalency-sensitive fingerprint of hydrogen bonds, provided that geometry and ensemble effects are included explicitly.