DFT Comparative Study of the Formamide and Fluoro-FormamideTautomers and Their Corresponding Transition States

doi:10.21203/rs.3.rs-164894/v1

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DFT Comparative Study of the Formamide and Fluoro-FormamideTautomers and Their Corresponding Transition States

https://doi.org/10.21203/rs.3.rs-164894/v1

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posted 25 Feb, 2021

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Seventeen tautomers of formamide [NH₃CO] and thirty one tautomers of fluoro-formamide [NH₂FCO] were examined by density functional theory (DFT) calculations. Five methods of DFT (B3LYP/ 6-311++G, B3LYP/ 6-311++G(2df,2p), B3LYP/cc-pVTZ, wB97X-D/6-311++G(2df,2p) and wB97X-D/ cc-pVTZ) were used to calculate the energies and optimized geometries of formamide tautomers. B3LYP/cc-pVTZ method was determined as the best method and used to calculate relative energies, optimized geometries, mulliken charge distributions, Gibbs free energies and enthalpy of formations of all proposed fluoro-formamide/fluoro-formamidic acid tautomers. Ten transition states of formamide tautomers and nineteen transition states of fluoro-formamide tautomers were determined by vibrational frequency calculations (they have an imaginary value of frequency). Using B3LYP/cc-PVTZ, the relatively small energies of formamide (A1) and formamidic acid (A6) (-446246 and -446193kJ.mol -1 , respectively) participate to it being more stable than other formamide/formamidic acid tautomers, as well as for fluoro-formamide (C1) (-706947 kJ.mol -1 ) and fluoro-formamidic acid (C3) (-706855 kJ.mol -1 ). The stability of fluoro-formamide at carbonyl and at nitrogen was discussed using two thermodynamic calculations: Enthalpy of formation [∆H(298)] and Gibbs free energy (∆G) values, positive ∆G values (59 to 114 kJ mol-1) for fluoro-formamidetautomerism confirms the stability of fluoro-formamide at carbonyl.

Computational Chemistry

Formamide Tautomer

Fluoro-formamide Tautomer

DFT calculations

Transition State

Gibbs free energy

Enthalpy of formation

Table I. Comparative bond lengths (Å) and angles (°) of the A₁ and A5 conformers

Tau.	Method	B3LYP/ 6-311++G	B3LYP/ 6-311++G (2df,2p)	B3LYP- cc-pVTZ	wB97X-D/ 6-311++G (2df,2p)	wB97X-D/ cc-pVTZ	B3LYP- 6-311++G (2df,2pd)^a	Micro-wave^b	X-ray^c	Δ%*	Δ%**
A₁	d C₄ = O₆	1.212	1.210	1.209	1.207	1.205	1.209	1.219	1.239	0.8	2.4
	d C₄ ̶ N₂	1.361	1.357	1.357	1.357	1.354	1.357	1.352	1.326	0.4	2.3
	d N₂ ̶ H₁	1.007	1.004	1.004	1.005	1.002	1.004	1.002	1.010	0.2	0.6
	d N₂ ̶ H₃	1.009	1.007	1.006	1.007	1.005	1.007	1.002	1.010	0.4	0.4
	d C₄ ̶ H₅	1.106	1.104	1.105	1.105	1.104	1.104`	1.098	1.090	0.6	1.4
	< O₆C₄N₂	124.93	124.88	125.01	124.78	124.86	124.9	124.7	124.9	0.3	0.09
	< H₃N₂C₄	119.44	119.44	119.37	119.20	119.24	119.4	118.5	118.0	0.7	1.2
	< H₁N₂C₄	121.40	121.38	121.36	121.42	121.35	121.3	120.2	119.0	1.0	2.0
	< H₅C₄O₆	12.67	122.63	122.81	122.64	122.74	122.6	122.6	-		-
	< N₂C₄H₅	112.30	112.49	112.18	112.58	112.40	112.5	112.7	116.0	0.5	3.3
	< H₁N₂H₃	119.16	119.18	119.27	119.38	119.42	119.2	121.5	-		-
A6	d C₁ ̶ O₃	1.347	1.345	1.346	1.337	1.338	1.362	-	-		-
	d C₁ = N₄	1.263	1.259	1.259	1.258	1.257	1.254	-	-		-
	d N₄ ̶ H₆	1.016	1.014	1.014	1.012	1.013	1.020	-	-		-
	d O₃ ̶ H₅	0.970	0.969	0.969	0.964	0.965	0.963	-	-		-
	d C₁ ̶ H₂	1.091	1.088	1.089	1.089	1.089	1.089	-	-		-
	< O₃C₁N₄	122.19	122.28	122.26	122.19	122.16	-	-	-		-
	< H₆N₄C₁	111.66	111.95	111.58	111.54	111.18	111.7	-	-		-
	< H₄₅O₃C₁	107.12	107.33	106.74	107.32	106.71	110.5	-	-		-
	< H₂C₁O₃	110.14	110.25	110.06	110.65	110.52	114.7	-	-		-
	< H₂C₁N₄	127.67	127.47	127.68	127.16	127.32	120.6	-	-		-

* Percentage of error (Δ%): between wB97X-D/cc-pVTZ (This Work) and Microwave.

** Percentage of error (Δ%): between wB97X-D/cc-pVTZ(This Work) and X-ray.

^a Ref. [26].

^b Ref. [28]

^c Ref. [29].

Table II not available with this version

Table III. Comparative bond lengths (Å) and angles (°) of the C1 and C3tautomersbased on B3LYP/cc-PVTZ method

Conformer	Method	B3LYP-cc-pVTZ	Conformer	Method	B3LYP- cc-pVTZ
C1	d C₁ = O₂	1.189	C3	d C₁ ̶ O₂	1.342
	d C₁ ̶ N₃	1.347		d C₁ = N₃	1.244
	d N₃ ̶ H₅	1.003		d N₃ ̶ H₅	1.015
	d N₃ ̶ H₄	1.003		d O₂ ̶ H₄	0.966
	d C₁ ̶ F₆	1.363		d C₁ ̶ F₆	1.344
	< O₂C₁N₃	129.06		< O₂C₁N₃	120.02
	< H₄N₃C₁	121.08		< H₅N₃C₁	111.30
	< H₅N₃C₁	118.68		< H₄O₂C₁	109.71
	< F₆C₁O₂	121.73		< F₆C₁O₂	110.55
	< N₃C₁F₆	109.21		< F₆C₁N₃	120.43
	< H₅N₃H₄	120.24

Table IV. Relative Energies (kJ.mol^-1) of fluoro-formamide tautomers (by B3LYP/cc-PVTZ method)

Energy from	∆E_SCF(kJ/mol)	∆H (0K)	∆H (298K)	∆G (298K)
C1	-706947	-706847	-706834	-706918
C2	-706892	-706794	-706782	-706863
C3	-706855	-706755	-706742	-706823
C4	-706854	-706754	-706742	-706823
C5	-706847	-706746	-706734	-706814
C6	-706836	-706735	-706723	-706804
C7	-706667	-706568	-706556	-706636
C8	-706654	-706556	-706543	-706625
C9	-706641	-706546	-706533	-706616
C10	-706642	-706544	-706531	-706613
C11	-706635	-706539	-706525	-706607
C12	-706629	-706534	-706521	-706603

Table V. Relative Energies (kJ.mol^-1) of fluoro-formamide transition states (by B3LYP/cc-PVTZ method)^a

Energy from	∆E_{SCF (kJ/mol)}	∆H (0K)	∆H (298K)	∆G (298K)
D1	97.06	91.58	91.51	95.74
D2	104.50	97.68	98.42	103.44
D3	107.31	101.81	101.78	105.97
D4	290.67	285.44	286.01	289.87
D5	295.35	290.68	291.09	294.55
D6	360.91	353.89	354.03	358.08
D7	370.54	364.52	364.66	368.54
D8	433.87	425.82	426.83	431.30
D9	49.45	43.55	44.10	44.85
D10	133.59	125.11	126.05	126.42
D11	142.00	134.83	135.55	136.26
D12	193.07	178.29	180.18	180.77
D13	210.50	195.34	197.28	197.81
D14	241.91	234.70	236.00	236.23
D15	247.43	241.41	242.17	242.82
D16	464.16	455.55	456.49	458.82
D17	485.96	476.36	477.37	479.57
D18	558.00	543.18	545.19	547.67
D19	561.26	545.61	547.66	550.06

Table VI. Gibbs free energy (∆G) (kJ/mol) for possible tautomerisms of fluoro-formamide and fluoro-formamidic acid (at 298 K)

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DFT Comparative Study of the Formamide and Fluoro-FormamideTautomers and Their Corresponding Transition States

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