Table 2. Weak Hydrogen Bond Parameters for Pent-4-ynoic Acid Solid.

pent-4-ynoic acid	da (Å)	C···Hb (Å)	C–H···Mc (°)	ΔHf(d)d (kcal/mol)	ΔHf(s)e (kcal/mol)	ΔHIPf (kcal/mol)	ΔHHg (kcal/mol)
PM3	2.5	2.5	174	–112.93	–117.43	–4.50	–2.25
		2.5
AM1	2.6	2.7	165	–113.67	–117.08	–3.41	–1.71
		2.6
experimental data48	2.69	2.76	155
		2.74
ab initio(48)	2.67	2.75	159				–2.1
		2.71

^aC-H···π (M) distance, where M is the middle point of the C≡C bond.

^bThe distance between the donating H atom and both C atoms in the C≡C bond.

^cThe weak hydrogen bond angle, where M is the middle point of the C≡C bond.

^dThe heat of formation for the isolated dimer unit.

^eThe heat of formation for the solid.

^fThe total interaction energy.

^gThe net interaction energy per hydrogen bond.