Table 2.

Local stretching force constants of the C=O bonds for acceptor molecules A–F in descending order.

No.	I-I⋯O=C	${\mathit{k}}_{\mathit{n}}^{\mathit{a}}$ (mdyn/Å)	Cl-I⋯O=C	${\mathit{k}}_{\mathit{n}}^{\mathit{a}}$ (mdyn/Å)
1	E1-1	10.327	E1-2	9.932
2	A	9.755	E2	9.905
3	F-1	9.000	F-2	8.807
4	C1-1	8.945	C1-2	8.579
5	C2-1	8.937	C2-2	8.575
6	B2	8.431	D	8.561
7	B1-1	7.917	B1-2	7.651

The local mode frequencies ${\omega }_n^a$ for C=O bond stretching can be calculated by ${\omega }_n^a=\sqrt{k_n^a}·497.6$, where $k_n^a$ is in the unit of mdyn/Å and the resulting vibrational frequency ${\omega }_n^a$ is in the unit of cm${}^{-1}$.