3.3 Low Surface-Coverage
3.3.1 Energies
For the low surface-coverage calculations, a periodic slab constructed from 4×5 units of the Cu(110) unit cell was used, with 4 Cu layers. The lower two were fixed to remain in the calculated Cu(110) bulk geometry, with the optB86b-vdW functional calculated lattice constant of 3.60 ˚A. This value is in good agreement with the experimental value of 3.61 ˚A. It was assumed that, in a similar fashion to the adsorption exhibited by 3CP on Cu(110), 4CT would bond through the carboxyl group, and the carboxylic acid would deprotonate. This assumption is supported by the RAIRS spectrum obtained as 4CT is deposited on Cu(110). This spectrum shows two characteristic peaks at 1393 cm−1 and 1367 cm−1. Work on 3CP by Robin et al. [50] showed similar peaks in the
RAIRS spectrum at 1450 cm−1 and 1396 cm−1, Robin et al. were able to assign these
peak at 1396 cm−1 corresponded to a combination of a NO stretch and COO symmet-
ric stretch. This peak disappeared for adsorption configurations bonded through the radical with the carboxylic acid group pointing away from the surface. The similarities between the RAIRS spectra of 4CT and 3CP provide a very strong indication that the molecule is bonding through the carboxyl group.
Illustrations of the various adsorption geometries of the low-coverage molecule, along with adsorption energies and the relative stabilities of the different geometries are given in Table 3.2. In the cases where a stable gas-phase conformer existed, this geometry was placed on an adsorption site and relaxed. Where no stable gas-phase conformer existed the starting geometries of the gas-phase calculations were used as shown in Table 3.1.
All systems were adsorbed through the carboxylate, as this was shown to be favourable by Robin and co-workers for 3CP. The adsorption sites chosen were the short bridge site (sb), where the carboxylate is bonded along a Cu close-packed row, and the long bridge site (lb), where the carboxylate bridges the Cu close-packed rows. The axial-A long bridge site was stabilised through the interaction of the NO group with the substrate. Since the distance between the surface and the NO group would be much larger for the equatorial conformers on the lb site the NO-substrate interaction would be much smaller and so the lb adsorption sites were not calculated for the equatorial conformers.
The adsorption energies of these systems are defined to be:
Eads(4CT/Cu) =E(4CT/Cu) +E(H/Cu)−(2E(Cu) +E(4CT(Eq.B))) (3.1)
WhereE(4CT/Cu) is the total energy of the molecule adsorbed on Cu(110),E(H/Cu) is the total energy of a hydrogen atom adsorbed on Cu(110),E(Cu) is the total energy of the clean Cu(110) surface, and E(4CT(Eq.B)) is the total energy of the 4CT gas- phase equatorial-B conformer (the most energetically favourable gas-phase conformer). The results show that the presence of the Cu surface changes the energetics of the different conformers substantially, with conformers that were unstable in gas-phase being stabilised. Three factors appear to dictate the stability of the various conformers
Table 3.2: Calculated relative stabilities of various conformers of 4CT adsorbed on Cu(110) at low surface-coverage. ‘(lb)’ indicates adsorption through the carboxylates via the long bridge site on Cu(110).
System Top Side
Adsorption Energy (meV) Relative Sta- bility (meV) axial-A (boat) -2692 0 axial-B (boat) -2182 510 axial-A (boat, lb) -2128 564 axial-A (chair, 1) -2084 608 axial-A (chair, 2) -1839 853 equatorial-B (chair) -1814 879 equatorial-A (chair) -1733 959 axial-B (chair) -1697 996 axial-A (chair, lb) -1600 1092
on the surface. First of all, it is generally the case that bonding through the carboxyl via the sb site is favoured over bonding via the lb site. Hence, the axial-A (boat) conformer is 564 meV more stable when bonded via the sb site than when bonded via the lb site, and the axial-A chair conformer is 239 meV more stable when bonded via the sb site as opposed to the lb site. This preference for bonding via the sb site is similar to behaviour seen in the work carried out on 3CP, where the preference for bonding via the sb site is 90 meV [50]. This preference for the sb site is explained by the fact that the O-O distance in the carboxyl group (2.2 ˚A) is much closer to the Cu-Cu distance in the close packed rows (2.5 ˚A) than to the Cu-Cu distance between rows (3.6 ˚A).
The second factor influencing the stabilities of the conformers is a favourable in- teraction between the methyl groups on the TEMPO ring and the Cu surface. This can be seen in the energetic favourability of the axial-A (chair, 1) conformation over the axial-A (chair, 2) conformation. These two conformations are distinguished by the angle the C4-COOH bond makes with the surface perpendicular; the axial-A (chair, 1)
molecule makes an angle of -11.8◦, whereas the axial-A (chair, 2) conformer makes an angle of 13.8◦, thus increasing the distance between the methyls and the Cu surface, resulting in the conformation becoming unfavourable by 245 meV.
The final factor influencing the energetics is a favourable interaction of the oxygen in the NO group with the Cu surface. When 4CT is in the axial-A (boat) configuration the NO oxygen and the oxygens in the carboxylate group can bridge the close packed Cu[1¯10] rows. Hence, the NO oxygen can sit on the short bridge adsorption site of the Cu(110) surface. With the axial-A (boat, lb) configuration the conformer is bonded via the lb site and the oxygen of the NO group sits on the lb site of the Cu(110) surface. In the axial-B (boat) conformation the match is not as convenient, and so the carboxylate group must deform slightly in order to ensure that the NO oxygen is in a position near the Cu(110) top site.
RAIRS studies of nitric oxide adsorbed on Cu(110) showed that it adsorbs on a bridge site [126, 127] and oxygen adsorbed on Cu(110) also adsorbs on the long-bridge site [128], which suggests the positioning of the NO group over a bridge site is energet-
ically favourable, in accordance with what is observed here, where this interaction is favourable enough to stablise the boat conformers over the chair.
This interaction between the surface and the NO group stabilises the boat conform- ers that are unstable in the gas-phase, including the lb bonded molecule, over all the chair-type molecules. The nature of this interaction, and the implications it has for the preservation of the NO radical, will be looked at in the next section.