3.1.3 Tetramer:
Similar to its trimeric counterpart, tetramer can also exist in both linear and cyclic forms. Six different linear conformers are possible for tetramer based on the relative orientation of two neighboring monomeric units, and have been named as CCC, CCT, CTC, TCT, CTT, and TTT. Calculations performed at the B3LYP level of theory showed all of the six clusters to be linear (Figure S3). However, when ωb97xd functional was used only two conformers (CTT and TTT) remained linear, while the remaining four were converted to closed form (Figure S4) during optimization. This indicates that consideration of long-range dispersion interaction is a must while working with weakly H-bonded molecular clusters.
In the case of the cyclic tetramer, four different conformers are possible based on the relative orientation of the free carbonyl groups; named as tetramer1, tetramer2, tetramer3, and tetramer4 (Figure S4). In tetramer1, all four free carbonyl groups are in the same direction while tetramer2 has two alternate free carbonyl groups on the same side. Tetramer3 has two neighboring free carbonyl groups in the same direction and the remaining two in the opposite direction. For Tetramer4 three free carbonyl groups are in the same direction.
Binding energies of all 10 possible conformers of tetramer are given in Table 1. Out of the 8 cyclic tetramers, tetramer CCC was found to be the most stable with binding energy -29.7 kcal mol-1, while tetramer1 was the least stable one with binding energy -25.1 kcal mol-1.Therefore, the binding energies of the cyclic tetramers vary by a significant margin (4.6 kcal mol-1). On the other hand, the two linear tetramers, which are energetically less stable than all the cyclic conformers are very close to each other; CTT (-24.1 kcal mol-1) being only 0.1 kcal mol-1 more stable than TTT (-24.0 kcal mol-1). If the prediction of the B3LYP level of theory is considered, which predicts six linear and four cyclic conformers, the relative energies of the cyclic forms vary by a large amount while the same for linear are pretty close together as can be inferred from in Table 1.