Ballistic Transport of Vibrational Energy through an Amide Group Bridging Alkyl Chains

Abstract

Transport of vibrational energy via linear alkyl molecular chains can occur efficiently and with a high speed. This study addresses the question of how such transport is changed if an amide group is incorporated in the middle of such chain. A set of four compounds, Amn-4, was synthesized such that an amide group is connected to two alkyl chains. The alkyl chain on one side of the amide, featuring 4, 7, 11, or 15 CH2 units, is terminated by an azido group, while the alkyl chain on another side is of fixed length with four methylene groups terminated with a methyl ester group. The energy transport in Amn-4 in CD3CN solution, measured by relaxation-assisted two-dimensional infrared spectroscopy, was initiated and recorded using various combinations of tags and reporters, which included N3 and CO stretching modes of the end groups and amide-I and amide-II modes at the amide. It was found that the transport initiated by the amide-I mode in the alkyl chain attached to CO side of the amide proceeds with a constant speed of 4.2 Å/ps, supported by the CH2 rocking band of the chain. The end group-to-end group energy transport times for compounds with uneven alkyl chain length fragments appears to be additive. The transport from either end group of the molecule started as ballistic transport. The passage through the amide was found to be governed by intramolecular vibrational relaxation steps. After it passed the amide group, the transport was found to occur with constant but different speeds, dependent on the passage direction. The transport toward the ester was found to occur with the speed of 4.2 Å/ps, similar to that for the amide-I mode initiation and supported by the CH2 rocking band. The transport toward the azido group occurred with the speed of 8.0 Å/ps, which matches the speed supported by the CC stretching band. The results suggest that, after the CO group initiation, the excess energy reaches the amide group ballistically, redistributes at the amide, and reforms a wavepacket, which propagates further with a high speed of 8 Å/ps. This observation opens opportunities of controlling the energy transport process in molecules by affecting the alien group via specific interactions, including hydrogen bonding.

Low-Temperature Vibrational Energy Transport via PEG Chains

“…Understanding the transport of vibrational energy in molecules is important for many fields including chemistry and biochemistry, , molecular electronics, and the development of novel materials. − Vibrational energy transport plays a critical role in essentially all chemical reactions, as thermal energy flows into the reactants and out of the products for reaction to occur. − A better understanding of these processes could lead to the development of more refined models for chemical reactivity and greater insight into chemical reaction pathways. The capability to modulate the energy flow and direct it can potentially enable selection of reaction outcomes …”

“…Recent studies of energy transport via polyethylene glycol (PEG) chains, performed at room temperature (RT) using RA 2DIR, showed that the ballistic transport regime is limited to distances of 6–8 PEG units, defined by the dephasing time of the chain states . For chain lengths exceeding the mean free path (MFP) length of the wavepacket, the transport follows the directed diffusion mechanism, where only forward wavepacket-scattering events deliver significant energy to the chain end, due to significant energy losses to the solvent. , An increased regularity of the chain, for example, by removing gauche chain kinks, may result in a MFP increase . At lower temperatures, the dephasing and relaxation processes are expected to become slower, increasing the MFP length and the transport efficiency.…”

Topology, Landscapes, and Biomolecular Energy Transport

“…1d). This corresponds to backbone propagation at v = 1.7 nm ps −1 , approaching ballistic transport velocities in biomolecular materials and alkyl chains [13,[26][27][28]44]. While this channel is weak, additional ballistic pathways may exist at lower group velocities in different vibrational bands [44,45], though these will inevitably be obscured by more prominent diffusive features.…”

“…The results are complex, suggesting a 'dynamical transition' temperature above which transport is enhanced [17][18][19][20]. Quantum and nonequilibrium molecular dynamics (NEMD) simulations support the presence of a transition in transport properties, and also suggest that a classical description is realistic [21][22][23][24] (unlike for small molecules [25][26][27][28][29]). However, both the nature of the transition and mechanism of transport remain unclear, with theory giving conflicting accounts [13,17,21,22,30].…”

Competition of Several Energy-Transport Initiation Mechanisms Defines the Ballistic Transport Speed

“… 9 The ballistic through-chain transport was initiated via excitation with a mid-IR photon, a vibrational mode at the end group, which then transferred its energy into the chain, initiating the transport. The energies of the end-group modes tested for ballistic transport range from 2100 cm –1 (azido group stretch) 1 to 1650–1750 cm –1 (carbonyl groups in carboxylic acid, ester, succinimide ester, and amide), 9 , 10 1500 cm –1 (amide II mode of an amide), 10 and ∼1300 cm –1 (azido group stretch). 9 It was found that the selection of the end-group mode used to initiate the transport determines which chain band transfers energy, thus determining the transport speed and efficiency.…”