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The conformational landscapes of diallyl ether (DAE) and diallyl sulfide (DAS) were investigated for the first time using rotational spectroscopy from 6-20 GHz supported by quantum mechanical calculations. A significant difference in the conformational distribution of these chalcogen-bridged compounds is predicted by theory at the B3LYP-D3(BJ)/aug-cc-pVTZ level as DAS has only one low energy conformer while DAE has up to 12 energy minima within 5 kJ mol -1 . This was confirmed by rotational spectroscopy as only transitions corresponding to the global minimum of DAS were observed while the spectrum of DAE was much richer and composed of features from the nine lowest energy conformers. To understand the effects that govern the conformational preferences of DAE and DAS, natural bond orbital and non-covalent interaction analyses were done. These show that unique orbital interactions stabilize several conformers of the ether making its conformational landscape more competitive than that of the sulfide. This is consistent with a bonding model involving decreased hybridization of the bridging atom as one moves down the periodic table which is confirmed by the experimental ground state structures of the lowest energy forms of DAE and DAS, derived using spectra of the 13 C and 34 S substituted species in natural abundance.

The conformational landscape of the monohydrated complex of N-allylmethylamine (AMA-w) was investigated for the first time using rotational spectroscopy from 8-20 GHz and quantum chemistry calculations. From a total of nine possible energy minima within 10 kJ mol -1 , transitions for the two most stable conformers of AMA-w were detected, and assigned aided by DFT and ab initio MP2 predictions. The observed rotational transitions displayed characteristic hyperfine splittings due to the presence of the 14 N quadrupolar nucleus. Quantum theory of atoms in molecules (QTAIM), non-covalent interaction (NCI) and natural bond orbital (NBO) analyses showed that the observed conformers of AMA-w are stabilized by two intermolecular interactions consisting of a dominant NH-O and a secondary C-HO hydrogen bond (HB) in which the water molecule acts simultaneously as a HB donor and acceptor. The HBs formed with water do not change the relative energy ordering of the most stable conformers of AMA but do affect the stability of higher energy conformations by disrupting the intramolecular forces responsible for their geometries. By comparing the intermolecular interaction energies with those of the monohydrates of the simplest primary (methylamine, MA), secondary (dimethylamine, DMA) and tertiary (trimethylamine, TMA) amines using symmetry-adapted perturbation theory (SAPT) calculations, we find that AMA forms the strongest bound complex with water. This is rationalized through the identification of subtle differences in stabilizing and destabilizing contributions across the amine-w series of complexes.