. 2015 May 13;16(5):10767–10796. doi: 10.3390/ijms160510767

Table 1.

Relative energy/free energy components for tautomeric/conformational isomers ^a.

Structures in Schemes	Gas		Dichloromethane			Water
Structures in Schemes	ΔE^g_int	ΔG^g_th	ΔE^s_int	ΔG^s_th	ΔG_solv	ΔE^s_int	ΔG^s_th	ΔG_solv
CH₂=CH–CH=NH (1)	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
TS (1 to 2)	113.9	−9.9 ^b	111.5	−10.1 ^b	7.8	110.3	−10.3 ^b	10.3
	123.2 ^c
CH₂=CH–CH=NH (2)	2.7	−0.2	2.9	−0.5	−1.8	2.9	−0.5	−2.0
	3.1 ^c		3.8 ^c		−2.1 ^d	3.7 ^c		−2.3 ^d
TS (1 to 3)
(95.4°, 96.0°, 96.3°) ^e	35.5	−3.5 ^b	35.4	−3.0 ^b	−0.6	35.5	−3.0 ^b	−0.9
(96.1°)	29.3 ^c
CH₂=CH–CH=NH (3)	13.4	−2.4	14.3	−4.0 ^b,f	1.3	13.0	−4.0 ^b,f	2.2
	11.4 ^c		10.1 ^c		2.6 ^d	9.7 ^c		3.2 ^d
CH₂=CH–CH=NH (4)	15.1	−2.8 ^b	16.1	−2.3 ^b	−8.7	15.1	−2.5 ^b	−1.4
						13.1 ^c		−0.9 ^d
O=CH–CH=NH (5)	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
O=CH–CH=NH (6)	2.9	−0.5	3.5	0.2	−1.3	3.8	0.0	−2.0
TS (6 to 7)
(87.3°, 88.2°, 88.4°) ^e	29.1	−3.4 ^b	29.9	−3.0 ^b	−3.3	30.4	−3.0 ^b	−4.5
(89.2°)	30.8
O=CH–CH=NH (7)	7.8	−0.1	8.0	1.0	0.4	8.2	0.4	0.2
	7.8 ^c		9.6 ^c		−1.9 ^d	10.5 ^c		−3.0 ^d
O=CH–CH=NH(8)	23.1	−5.4 ^d	26.8	−3.6 ^b	−10.6	29.0	−3.3 ^b	−14.9
	24.4		32.5 ^c		−16.2 ^d	36.1 ^c		−22.3 ^d
O=C=CH–NH₂ (9)	20.8	−1.0	19.7		7.4	19.4	−0.8 ^b	8.2
	35.1		32.0 ^c		9.2 ^d	31.4 ^c		10.3 ^d
HN=CH–CH=NH (10)	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0
HN=CH–CH=NH (11)	4.6	−2.3 ^b	5.8	−2.9 ^b	−3.4	6.5	−3.0 ^b	−4.8
	6.2 ^c		8.7 ^c	−2.7 ^b	−5.0 ^d	9.8 ^c	−2.9 ^b	−6.8 ^d
TS (11 to 13)
(92.7^o, 93.2^o, 93.4°) ^e	32.6	−3.6 ^b	34.3	−2.8 ^b	−5.2	35.2	−3.0 ^b	−7.2
(93.6°)	31.3
HN=CH–CH=NH (13)	11.8	−2.8 ^b	13.0	−3.3 ^b	−1.5	13.6	−4.1 ^b	−2.3
	10.6 ^c		13.9 ^c		−4.5 ^d	15.3 ^c		−6.2 ^d
HN=CH–CH=NH (12)	6.9	−1.1	7.8	−1.4	−3.6	8.2	−1.7	−4.8
	9.4 ^c		11.6 ^c		−4.9 ^d	12.6 ^c		−7.1 ^d
HN=CH–CH=NH (14)	16.1		16.7	−0.9 ^b	−3.0	17.0	−2.2 ^b	−4.0
			20.8 ^c		−5.7 ^d	21.7 ^c		−7.2 ^d
TS (10 to 15)
(86.3°, -, 93.5°) ^e	35.2	−4.0 ^b				38.3	−3.8 ^b	−9.8
(83.6^o)	32.3
HN=CH–CH=NH (15)	28.8		33.3	−4.7 ^b	−12.8	36.2	−5.4 ^b	−18.5
			36.7 ^c		−17.4 ^c	40.8 ^c		−24.4 ^d
HN=C=CH–NH₂ (16)	49.2		49.8		0.1	50.2	−7.5 ^g	−0.8
	64.0 ^c
H₂N–C≡C–NH₂ (17)	98.6	−7.4 ^g	99.2		−2.6	99.2	−5.0 ^g	−4.0

^a Values in kJ/mol. For structure numbers in parentheses, see Scheme 1 and Scheme 2. Geometries were optimized at the B97D/aug-cc-pvtz and MP2/aug-cc-pvtz levels in the indicated environment. ΔE_int and ΔG_solv values (upper rows) from B97D/aug-cc-pvqz single point calculations; ^b –RT ln2 = −1.7 kJ/mol is included in ΔG_th for the entropy of mixing for the TS antipodes or for a symmetry number of 2 for the reference structure; ^c CCSD(T)_CBS//MP2/aug-cc-pvtz energies; ^d MP2/aug-cc-pvtz value; ^e Values in parentheses for a TS (transition state) stand for the X=C–C=N torsion angles increasing in the gas, CH₂Cl₂, and water series (X = CH₂, O, NH). The energy/free energy parameters are provided with respect to the corresponding data of the CH₂CHCHNH (1), OCHCHNH (5) and NHCHCHNH (10) conformers, respectively; ^f A small imaginary frequency for the lowest energy out-of-plane torsion remained through the energy minimization even using the analytical second derivative optimization. ΔG^s_th was estimated by using the corresponding water frequency (65 cm⁻¹), for which the normal coordinate was extremely similar. For all other water vibrations, the frequencies and the normal coordinates were very similar and deviated by 2–4 cm⁻¹ for the low-frequency vibrations. The largest deviation has been found at 6 cm⁻¹ above 2000 cm⁻¹; and ^g −2RT ln2 = −3.4 kJ/mol is included in the ΔG_th because of the rotational symmetry number of 2 for the reference tAA (9) form with C_2h symmetry and due to the entropy of mixing for antipodes. TS: transition state.