Investigation of L(+)-Ascorbic acid with Raman spectroscopy in visible and UV light

Figures & data

Figure 1. l-Ascorbic acid, C6H8O6 (systematic name (5R)-5-((1S)-1,2-dihydroxyethyl)-3,4-dihydroxyfuran-2(5H)-one), or here just AH₂. The endiol group is the ‒C(OH)˭C(OH)‒ part of the ring with the double bond between C2 and C3. Other atom numbering systems have been used in the literature (4, 24–26, 43). The dihedral (torsional) angles are defined as shown for atoms A–B–C–D, with A nearest the observer, who is looking down the B–C bond. The angle is that between the projected bonds AB and CD (+ clockwise).

Figure 2. Raman cell for titration experiments in oxygen-free pure argon atmosphere.

Table 1 Selected structure data in solved crystal structures compared to DFT calculated^a results for AH₂, AH⁻, and A²⁻

		AH2 molecule					AH^− Ion modeling		A^2− Di-ion modeling
		Conformer like		Minimum			Minimum		Minimum
		in crystal^c		conformers		NaAH or	conformers^c		conformers
	AH2	________________		________________		LiAH crystal	________________		________________
	crystal	In gas	In water	In gas	In water	X-ray	In gas	In water	In gas	In water	In water
	structure^b	phase	(PCM)	phase^d	(PCM)^e	structure^f	phase	(PCM)^e	phase	(PCM)	(PCM)^e
Column 1	2	3	4	5	6	7	8	9	10	11	12
Structure spectra Bond distances (Å)			Figure 3 Figure 11		Figure 4 Figure 12			Figure 5 Figure 13		Figure 8	Figure 9 Figure 14
C1–O1	1.216(2)	1.205	1.205	1.204	1.216	1.233(6)	1.226	1.229	1.221	1.225	1.234
C2–O2	1.361(2)	1.355	1.355	1.355	1.355	1.384(5)	1.383	1.381	1.283	1.302	1.317
C3–O3	1.326(3)	1.350	1.350	1.343	1.334	1.287(5)	1.264	1.268	1.368	1.351	1.294
C1–O4	1.355(2)	1.364	1.364	1.377	1.366	1.358(5)	1.395	1.389	1.394	1.383	1.416
C4–O4	1.444(2)	1.445	1.445	1.450	1.449	1.448(5)	1.439	1.443	1.446	1.447	1.444
C5–O5	1.427(3)	1.419	1.419	1.413	1.419	1.410(5)	1.433	1.434	1.387	1.402	1.428
C6–O6	1.431(4)	1.428	1.428	1.424	1.426	1.423(5)	1.431	1.429	1.435	1.432	1.433
C2–C3	1.338(2)	1.338	1.338	1.340	1.347	1.373(6)	1.383	1.381	1.381	1.373	1.398
C1–C2	1.452(3)	1.464	1.464	1.457	1.454	1.416(6)	1.421	1.419	1.506	1.482	1.445
C3–C4	1.493(3)	1.502	1.502	1.499	1.500	1.516(6)	1.538	1.532	1.492	1.496	1.518
C4–C5	1.521(4)	1.540	1.540	1.543	1.541	1.536(6)	1.535	1.536	1.551	1.547	1.544
C5–C6	1.521(3)	1.525	1.525	1.538	1.533	1.503(6)	1.531	1.529	1.555	1.546	1.534
Angles (°)
C4–O4–C1	109.1(2)	109.7	109.7	109.5	109.6	108.0(3)	107.4	108.0	108.1	108.1	107.5
O4–C1–C2	109.5(2)	108.6	108.6	108.3	109.2	110.6(4)	110.0	109.9	111.2	111.5	111.4
C1–C2–C3	107.8(2)	108.5	108.5	108.8	108.0	109.5(4)	110.9	110.6	103.2	104.1	106.4
C2–C3–C4	109.5(2)	109.3	109.3	109.5	109.6	105.8(4)	104.6	105.4	112.6	111.9	109.2
C3–C4–O4	104.0(2)	104.0	104.0	103.9	103.6	105.2(4)	106.9	106.0	104.6	104.2	105.4
O4–C1–O1	121.4(3)	124.5	124.5	123.6	121.8	120.4(4)	121.2	120.5	118.6	119.1	117.1
O1–C1–C2	129.1(2)	126.9	126.9	128.1	129.0	129.0(4)	128.8	129.5	130.2	129.4	131.5
C1–C2–O2	124.6(2)	122.9	122.9	123.1	124.8	121.6(4)	120.5	122.7	123.3	123.6	124.2
O2–C2–C3	127.5(2)	128.6	128.6	128.1	127.2	128.7(4)	128.5	126.7	133.5	132.3	129.4
C2–C3–O3	133.5(2)	131.2	131.2	131.3	132.0	131.3(4)	134.7	133.0	129.9	130.2	130.9
O3–C3–C4	117.1(2)	119.5	119.5	119.3	118.4	122.9(4)	120.7	121.6	117.5	117.9	119.9
C3–C4–C5	114.8(2)	114.4	114.4	114.9	114.2	116.1(4)	112.3	112.8	114.6	112.4	113.5
O4–C4–C5	110.4(2)	110.7	110.7	110.3	111.1	110.3(4)	112.1	111.8	114.3	114.7	111.5
C4–C5–O5	111.7(2)	112.8	112.8	111.6	111.7	113.4(4)	108.6	108.3	109.6	107.9	108.1
C4–C5–C6	112.7(2)	111.6	111.6	111.9	114.2	110.1(4)	114.3	114.1	113.5	113.1	113.5
O5–C5–C6	106.9(2)	106.3	106.3	110.4	112.1	108.5(4)	109.2	109.7	108.8	109.3	110.6
C5–C6–O6	108.0(2)	107.3	107.3	111.4	114.0	108.5(3)	109.6	110.2	106.8	107.5	112.3
Dihedral angle^g (°)
Angle χ1	−2.90	−0.6	−0. 6	1.4	16.2	−110.5	0.2	0.3	No H2	No H2	No H2
C1–C2–O2–H2	−35.10					135.2 (Li salt)
Angle χ2	−3.80	−3.4	−3.5	4.0	01.9	No H3	No H3	No H3	No H3 (Ho5 160.7)^h	No H3 (HO5 156.2)^h	No H3
C2–C3–O3–H3	−4.10
Angle χ3	66.60	57.2	57.2	48.9	58.9	55.9	−57.7	−59.60.	−52.6	−49.6	−52.80
C3–C4–C5–O5	50.80					62.6 (Li salt)
Angle χ4	−67.60	−70.9	−71.0	94.4	74.4	−61.4	36.7	39.50.	28.8	28.3	31.2
C4–C5–O5–HO5	−99.10					−76.6 (Li salt)
Angle χ5	66.50	61.1	61.1	−78.6	−72.40	165.2	−173.6	−177.40.	−159.2	−159.3	73.1
C4–C5–C6–O6	64.75					–176.1 (Li salt)
Angle χ6	170.40	−168.5	−168.4	65.5	84.3	164.3	42.4	48.80.	25.5	28.0	−49.40
C5–C6–O6–HO6	−151.60					162.1 (Li salt)
Dipole moment (D)		3.225	3.224	3.505^d	5.565		2.060	2.731	5.251	9.123	13.527
Total energy (Ha)		−684.9891598	−684.9891598	−684.9945050^d	−685.0346989		−684.4801816	−684.5608318	−683.7741241	−684.0558907	−684.0658267
Root mean squareGradient norm		0.0000058	0.0000117	0.0000041	0.0000041		0.0000186	0.0000056	0.0000057	0.0000100	0.0000104

^aCalculated in this work by application of the DFT/B3LYP/6-311++G(d,p) basis set.^bDiffraction results by Hvoslef (25). Average of values for two individual AH₂ molecules. Standard deviations in parentheses. Dihedral angles are given for both molecules, calculated by us (note that his coordinates were given in a left-handed coordinate system).^cStarting from Hvoslef's average structures (Hvoslef (10) or Hvoslef (25)).^dOur values starting from most stable gas molecule. Similar results were found (with other definitions of atom numbers and signs) by Singh et al. (53) and Yadav et al. (54), who used Gaussian 03 and DFT/B3LYP with the smaller 6-31++G** basis set. Their energy and dipole moment were given as −684.99328 Ha and 3.325 Debye.^eThe global minima in the water-phase model (most stable conformations) are shown in boldface.^fDiffraction results by Hvoslef (10). Dihedral angles were calculated by us (note that his coordinates were given in a left-handed coordinate system). Dihedral angles in italics are for the analogous Li⁺ salt that also has only one individual AH⁻ ion in its structure (31).^gDihedral angles are defined in Figure 1.^hHo5 is bound to O3 and the angle given is χ₂ = C2–C3–O3–H_O5.

Figure 3. Ascorbic acid molecule, AH₂, as optimized by us starting from Hvoslef's average crystal structure (25). Calculations done with the Gaussian minimization procedure using DTF/B3LYP/G6-311++(d,p) with the water as solvent in the PCM solvation modeling. This is a local but not the global minimum of the single-molecule.

Figure 4. Most stable structure of the ascorbic acid molecule, AH₂, as optimized by Gaussian minimization with the DTF/B3LYP/G6-311++(d,p) procedure and using water as the solvent in the PCM solvation modeling. The structure is reminiscent of that found in the gas phase by Milanesio et al. (26), Singh et al. (53), and Yadav et al. (54).

Table 2 Intramolecular hydrogen bond distances for AH₂, AH⁻, and A²⁻ molecules in different minimized^a conformations

	AH2 molecule				AH^− anion				A^2− anion
	Conformer like		Other minimum		Starting from		Other minimum		Starting from		Other minimum
	in crystal^b		conformer		conformer like		conformers ^b		conformer like		conformers^b
	In	In	In	In	Hvoslef^b	Yadav^c	In	In	Hvoslef^b	Yadav^c	In	In
	gas	water	gas	water	In water	In water	gas	water	In water	In water	gas	water
	phase	(PCM)	phase^c	(PCM)^d	(PCM)	(PCM)	phase	(PCM)^d	(PCM)^d	(PCM)	phase	(PCM)
Column 1	2	3	4	5	6	7	8	9	10	11	12	13
Structure spectra Bond distances (Å)		Figure 3 Figure 11		Figure 4 Figure 12	Figure 6	Figure 7		Figure 5 Figure 13	Figure 9 Figure 14	Figure 10		Figure 8
O1 … .H2	2.484	2.484	2.516	2.655	2.528	2.525	2.398	2.515	No H2	No H2	No H2	No H2
O2 … .H3	2.725	2.725	2.714	2.751	No H3	No H3	No H3	No H3	No H3	No H3	No H3	No H3
O3 … .Ho5	3.083	3.083			1.826		1.795	1.873	1.727		1.008	1.059
Ho5…O5					0.982				0.993		1.687	1.473
O3…Ho5…O5					2.724				2.658		2.632	2.487
O4 … .Ho5			2.944	2.676		2.852
O4 … .Ho6			2.170	2.697	2.095	2.061			2.009	1.918
O5 … .Ho6							2.187	2.319			1.800	1.889
Ho6–O6												0.982
O5 … .Ho6–O6												2.600
O6 … .Ho5			2.178	2.541		2.274
O6 … .Ho6									0.969
Total energy (Ha)	−684.9891598	−684.9891598	−684.9945050	−685.0346989	−684.5596177	−684.5588461	−684.4801816	−684.5608318	−684.0658267	−684.0613658	−683.7741241	−684.0558907

^aCalculated in this work by application of the DFT/ B3LYP/6-311++G(d,p) basis set.^bStarting from Hvoslef's average structures; see Hvoslef (10) or Hvoslef (25).^cOur values starting from most stable gas molecule. Quite similar results were found (with other definitions of atom numbers and signs) by Singh et al. (53) and Yadav et al. (54), who used Gaussian 03 and DFT/B3LYP with the 6-31++G** basis set. Their energy and dipole moment were given as −684.99328 Ha and 3.325 Debye.^dThe global minima in the water-phase model (most stable conformations) are shown in boldface.

Figure 5. Optimized structure of the AH⁻ ion in the PCM water model as obtained after Gaussian minimization based on our guess. This global minimum conformer of AH⁻ has three internal hydrogen bonds (Table 2).

Figure 6. Optimized structure of the AH⁻ ion in the PCM water model as obtained after Gaussian minimization based on the crystal structures solved by Hvoslef (10). This local minimum conformer of AH⁻ has three internal hydrogen interactions but the energy is not as low as that of Figure 5 (Table 2).

Figure 7. Optimized structure of the AH⁻ ion in the PCM water model as obtained after Gaussian minimization based on the results of Singh et al. (53) and Yadav et al. (54) for AH₂. This local minimum conformer of AH⁻ has four internal hydrogen interactions but the energy is not as low as that of Figure 5 (Table 2).

Table 3 Calculated dihedral (torsion) angles χ₁, χ₃–χ₆ for the ascorbate ion minimum energy conformer (Figure 5) compared with similar angles in selected crystal structures^a

Dihedral angles in degrees, defined in Figure 1	Ascorbate minimum conformer	Lithium ascorbate (31)	Sodium ascorbate (10)	Calcium di-ascorbate dihydrate (27–29)		Strontium diascorbate dihydrate (30)		Thallium ascorbate (32)
χ1 C1–C2–O2–H2	0.3	135.2	−110.5	−154.8	−118.3	−142.3	−120.7	Missing	Missing
χ3 C3–C4–C5–O5	−59.6	62.6	55.9	169.9	−69.2	170.4	−69.9	55.6	61.1
χ4 C4–C5–O5–HO5	39.5	−76.6	−61.4	−39.9	−47.6	−44.5	−58.7	Missing	Missing
χ5 C4–C5–C6–O6	−177.4	−176.1	165.2	−74.4	−73.2	−70.6	−72.5	166.5	64.0
χ6 C5–C6–O6–HO6	48.8	162.1	164.3	Missing	141.2	97.9	135.6	Missing	Missing

^aThe angles were calculated by us using data from the CCSD database and their Mercury program. Data for some hydrogen atoms are missing in the database.

Figure 8. Optimized structure of the A²⁻ ion in the PCM water model as obtained after Gaussian minimization based on a guess. This local minimum conformation of A²⁻, rather similar to a similar gas-phase conformer (Table 1), has two internal hydrogen interactions but the energy is not as low as that in Figure 9 (Table 2).

Figure 9. Optimized structure of the A²⁻ ion in the PCM water model as obtained after Gaussian minimization starting from a guessed structure derived from the crystal results by Hvoslef (10, 25). This global minimum conformer of A²⁻ has two internal hydrogen bonds (Table 2).

Figure 10. Optimized structure of the A²⁻ ion in the PCM water model as obtained after Gaussian minimization based on a guessed structure derived from the results for AH2 of Singh et al. (53) and Yadav et al. (54). This local minimum conformer of A²⁻ has one internal hydrogen interaction and the Ho5 is interacting with the π electrons above C3, but the stabilization is not as good as in the conformer of Figure 9 (Table 2).

Figure 11. Raman and infrared spectra calculated for the conformer of ascorbic acid AH₂ simulated in water solution for the local minimum conformation as shown in Figure 3.

Table 4 Selected wavenumber data^a for IR absorption and Raman scattering strong bands of AH₂, AH⁻, and A²⁻ compounds dissolved in water. Data were calculated by the Gaussian 03/DFT/B3LYP/6-311++G(d,p)/PCM

Modeling	AH2						AH^−			A^2−
	Conformer like in crystal			Global minimum conformer^b			Global minimum conformer			Global minimum conformer
Total energy (Ha)	−684.9891598			−685.0346989			−684.5608318			−684.0658267
	Structure Figure 3 Spectra Figure 11			Structure Figure 4 Spectra Figure 12			Structure Figure 5 Spectra Figure 13			Structure Figure 9 Spectra Figure 14
Assignment^b Brief description of normal modes	Mode no. (cm^−1)^c	IR (km/mol)	Raman (Å^4/AMU)	Mode no. (cm^−1)	IR (km/mol)	Raman (Å^4/AMU)	Mode no. (cm^−1)	IR (km/mol)	Raman (Å^4/AMU)	Mode no. (cm^−1)	IR (km/mol)	Raman (Å^4/AMU)
O6–H str	54: 3861	57	122	54: 3505^d	250^d	165^d	50: 3771	83	105	48: 3708	260	145
O5–H str	53: 3842	33	63	53: 3491	209	126	49: 3455	536	179	47: 3206	1003	201
O3–H str	52: 3796	101	100	51: 3307	563	286	—	—	—
O2–H str	51: 3753	120	81	52: 3360	515	300	51: 3776	117	210
C6–H asym str	50: 3072	26	50
C4–H str	49: 3069	3	131
C6–H str				50: 3075	29	267	48: 3104	36	147	46: 3086	71	246
C6–H sym str	48: 3023	41	141	49: 3013	50	574
C5–H str	47: 3012	16	91	48: 2994	1	253
C4–H str				47: 2978	3	412
C4–H & C5–H str ooph							46: 3043	50	172
C6–H str							45: 3010	48	284
C4–H, C5–H, C6–H str										45: 3016	241	823
C4–H, C5–H, C6–H str										44: 3005	75	582
C4–H, C5–H, C6–H str										43: 2997	39	287
C1 C2 C3 ring breath	46: 1829	367	18	46A: 1784	266	9	44A: 1746	203	57	42A: 1699	602	99
C1–O1, C2–C3 str	45: 1775	432	98	45B: 1699	1610	469
C3–C4, C1–C2 str, ring OH bend	42: 1440	48	7	43C: 1433	103	27
C1–O1, C2–C3, C2–O2 str, O3–H–O5 str							43B: 1604	2426	89	41: 1550	454	62
C–H bend	44: 1521	6	7				41: 1474	307	37
C6H2 sci, O3–H–O5 bend										40: 1498	454	101
C5–C6 str, C–H bend	43: 1458	3	1
C–H, O–H bend, C–C str	41: 1399	7	1							39B: 1495	1522	273
C5–C6 str, C6–O6–H bend										38: 1432	40	12
C2–O2 & C3–O3 str							39C: 1415	568	112
C3–C4 str, C–H bend	40: 1392	16	8							36C: 1369	560	100
C1–C2, C3–C4 str, ring OH bend	39: 1364	20	4
C2–O2, C4–C5 str	38: 1331	103	13	37: 1330	43	44
C3–O3 str, H4 def	37: 1309	161	1
C–H, O–H bend	36: 1295	6	10	36: 1295	514	16				37: 1396	32	20
C–H, O–H bend	35: 1284	58	17	35: 1285	134	6	35: 1324	150	1	34: 1321	355	109
C1–C2, C2–O2 & C3–O3 str										35: 1345	389	8
C3–C4 str, C–H bend							33: 1255	81	52	32: 1272	294	88
C–H, O–H bend	34: 1248	8	4	34: 1247	16	42				33: 1304	222	12
C–H bend	33: 1185	77	1	33: 1207	65	10				30: 1193	112	17
C1–C2, C3–O3, C3–C4 str	32: 1171	141	9	32: 1169	253	30				31: 1232	83	17
C1–O4, C3–C4, C2–O2 str	31: 1126	195	8	31: 1110	446	51
C4–C5, C5–O5, C4–O4 str	30: 1118	90	9	29: 1090	95	13				27: 1067	5	11
C2–O2, C4–O4 & C1–O4 str	29: 1075	65	1				29: 1089	278	18
C1–C2, C4–C5, C5–C6 str										29: 1151	106	18
C6H2 rock, C4–C5 str										28: 1103	271	15
O4–C4, C5–C6, C6–O6 str	28: 1067	55	4	30: 1099	242	20
C1–O4, C3–C4, C6–O6 str	27: 1043	76	3
O3–H–O5 bend
C5–C6 str	26: 1005	11	5
C6–O6 & C5–O5 str							26: 1036	201	7	24: 1028	232	4
C4–O4 & C5–O5 str	25: 991	41	2	27: 1036	159	3	25: 1030	353	7	26: 1057	111	3
C4–O4 & C6–O6 str				26: 962	47	5				25: 1032	164	15
C4–C5 & C2–O2 str	24: 866	15	3
C5–O5 str							23: 860	76	5	22: 871	9	15
C4 oopl bend	23: 822	13	7	24: 840	55	12	23: 822	13	7	21: 789	17	23
C5–O5 tors										20: 759	151	2
Ring def	20: 690	4	5	19: 632	10	21				19: 729	14	6
C4–C5 str, ring def	19: 625	15	13	18: 601	15	10				18: 716	15	26
Ring def	18: 579			17: 572	23	14				17: 688	25	13
O1–C1–O4 bend										16: 607	16	3
C6–O6 tors				15: 403	384	3	13: 420	145	2
Ring def										15: 578	20	49
C5–O5, C6–O6 tors, C3–O3 def				14: 377	98	1
C2–O2 & C3–O3 def	13: 293	189	1	12: 355	169	2				11: 391	39	1
O5–C5–C6 bend										9: 351	30	5
C5–O5 & C6–O6 tors	7: 218	28	1
Ring O–C ipl bend										7: 324	8	19
C2–O2 tors				5: 149	200	2	7: 217	123	1
O3–H–O5 str										5: 223	35	5

^aCalculated in this work by application of the 6-311++G(d,p) basis set. For structures see Table 1.^bAbbreviations for approximate vibrations: asym = asymmetric, bend = bending, breath = breathing, def = deformation, iph = in phase, ooph = out of phase, rock = rocking, sci = scissoring, str = stretching, sym = symmetric, tors = torsion (rotation). For modes A, B, and C see text and Figure 25.^cSee also values calculated by Shimada et al. (46) (reported in their table 2, column 5).^dFor example, for mode 54 (O3–H3 stretching) in the gas medium, Singh et al. (53) and Yadav et al. (54) got a wavenumber of 3798 and IR and Raman signals of 109 km/mol and 101 Å⁴/AMU. We got identical values for that medium.

Table 5 Selected model results for ions AH⁻ and A²⁻ in water (PCM)^a,b

	AH^− ion conformers			A^2− di-ion conformers
	Local minimum, starting from Hvoslef ^c	Local minimum, starting from Yadav^d	Global minimum	Global minimum, starting from Hvoslef ^c	Local minimum, starting from Yadav^d	Other local minimum
Figures	Structure Figure 6	Structure Figure 7	Structure Figure 5 Spectra Figure 13	Structure Figure 9 Spectra Figure 14	Structure Figure 10	Structure Figure 8
Dihedral angle χ1 C1–C2–O2–H2	1.2	−0.2	0.3	No H2	No H2	No H2
Dihedral angle χ2 C2–C3–O3–H3	No H3	No H3	No H3	No H3	No H3	No H3 (Ho5 156.2)^e
Dihedral angle χ3 C3–C4–C5–O5	−54.9	52.0	−59.6	−52.8	53.0	−49.6
Dihedral angle χ4	35.4	85.6	39.5	31.2	−26.3	28.3
C4–C5–O5–HO5
Dihedral angle χ5	75.5	−74.1	−177.4	73.1	−63.1	−159.3
C4–C5–C6–O6
Dihedral angle χ6	−52.0	57.4	48.8	−49.4	34.2	28.0
C5–C6–O6–HO6
Dipole moment (Debye)	2.258	7.006	2.731	13.527	16.076	9.123
Total energy (Ha)	−684.5596177	−684.5588461	−684.5608318	−684.0658267	−684.0613658	−684.0558907
Root mean square gradient norm	0.0000061	0.0000022	0.0000056	0.0000104	0.0000197	0.0000100
OH str mode no. Wavenumber Intensity Assignment^f	51: 3778 IR 118, Ra 199 O2–H2 str	51: 3780 IR 111, Ra 207 O5–H O5 str + O2–H2 str	51: 3776 IR 117, Ra 210 O2–H2 str	48: 3708 IR 260, Ra 146 O6–HO6…O4 str^g	48: 3708 IR 255, Ra 408 O5–HO5 …O3 str^g	48: 3708 IR 260, Ra 145 O6–H5 str
OH str mode no. Wavenumber Intensity Assignment^f	50: 3760 IR 182, Ra 124 O6–HO6…O4+π str^g	50: 3766 IR 95, Ra 77 O5–H O5 str + O6–HO6…O4+π str^g	50: 3771 IR 83 Ra 105 O6–HO6 str	47: 3206 IR 1003 Ra 210 O5–HO5…O3 str^g	47: 3673 IR 384 Ra 169 O6–HO6…O4+π str^g	47: 3206 IR 1003 Ra 201 O5–H5 str
OH str mode no. Wavenumber Intensity Assignment^f	49: 3432 IR 611, Ra 179 O5–HO5…O3 str^g	49: 3734 IR 200, Ra 145 O6–HO6…O4+π^g	49: 3455 IR 536, Ra 179 O5–HO5…O3 str^g	No H2 present	No H2 present	No H2 present

^aCalculated in this work by application of the DFT/B3LYP/6-311++G(d,p) model. For structure data see Table 1.^bDihedral angles are defined in Figure 1. Dihedral angles are in degrees. Global minimum values are shown in boldface.^cStarting from Hvoslef's average structures (Hvoslef (10) or Hvoslef (25)), deprotonating H3 and for A²⁻ also H2.^dStarting from results by Singh et al. (53) and Yadav et al. (54), with other atomic number definitions and deprotonating H3 and for A²⁻ also H2.^eHo5 is bound to O3 and angle given is χ₂ = C2–C3–O3–H_O5.^fFor modes calculated frequencies are given in cm⁻¹. IR and Raman signals are given in km/mol and Å⁴/AMU, respectively.^gInfluenced by hydrogen bonding.

Figure 12. Raman and infrared spectra calculated for the most stable ascorbic acid AH₂ simulated in water solution for the conformation as shown in Figure 4.

Figure 13. Raman and infrared spectra calculated for the ascorbate ion AH⁻ simulated in water solution for the conformation as shown in Figure 5.

Figure 14. Raman and infrared spectra calculated for the ascorbate di-anion A²⁻ simulated in water solution for the conformation as shown in Figure 9.

Figure 15. Reference Raman spectra for AH₂ and NaAH solids. Measurement details: Laser, 532 nm. Power level, ∼200 mW. Slit width, ∼8 cm⁻¹. Accuracy, ± 1 cm⁻¹. Assignments are given in Table 6. Insert shows details of the range from 1800 to 1400 cm⁻¹.

Table 6 Measured Raman spectral bands (in cm⁻¹) and assignments for ascorbic acid and sodium ascorbate solids, compared to literature values (39, 44–46, 48). Some data can also be found in Jehlička et al. (47)

Ascorbic acid						Sodium ascorbate
This work Figure 15, Figure 16	Hvoslef and Klæboe (39)	Panicker et al. (45)	De Gelder et al. (44)	Shimada et al. (46)	Saraiva et al. 300K (48)	This work Figure 15, Figure 19	Hvoslef and Klæboe (39)	Assignments
3528 w				3526	3535			O–H str
3411 w				3410	3419	3304 m		O–H str
3317 vw					3324	3247 w		O–H str
3211 vw								O–H str
3001 w	3002 m	3004 w		3003	3010			O–H + C–H str
2977 w	2978 m				2985	2979 m	2978 s	C–H str
						2962 m	2965 m	C–H str
							2954 m
						2950 m	2940 w	C–H str
2917 s	2916 vs	2919 s		2917	2924	2924 vs	2925 vs	C–H str
	2903 vs
2866 w	2866 w	2879 wsh			2870	2892 w		C–H str
1746 w	1754 w	1758 w			1755	1703 s	1704 vs	C˭C + C˭O str A
1667 vs	1667 vs	1661 vvs	1667 vs	1667	1674	1598 s	1598 vsbr	C˭C + C˭O str B
1653 vs	1654 vs		1653 vs	1652	1659			C˭C str
1497 m	1498 m	1484 m	1498 m	1497	1501			CH2 sci
	1487 w				1464	1483 w	1482 s	CH bend
						1359 vw	1360 m
1450 vw	1450 w	1452 w			1452	1330 w	1330 s	CH2 bend (sci)
1371 w	1372 w 1344 vw	1371 w		1371	1375	1305 w	1305 s	ring def, CH2 wag, C–O–H bend
1320 s	1321 s	1323 s	1319 s		1324	1277w	1275 s	CH2 bend (wag) C ?
1295 w	1297 m		1296 m		1300	1252 vw	1247 wbr
1256 s	1258 s	1258 s	1256 s	1256	1261	1236 vw	1236 m	C–O–H bend (twi)
1226 w	1226 w				1227	1155 vw	1156 m
1199 w	1200 m	1193 w			1202	1126 w	1127 m	C–C(–O)–O str
1128 s	1131 vs	1113 s	1130 vs	1129	1133			C–O–C str, ring def
	1114 vw				1121
1066 w	1066 m	1081 w			1073	1078 w	1077 s	C–O–C str, C–O–H bend
	1039 vw	1048 m			1037	1048 m	1048 s	C–O–C str
1028 m	1026 s		1025 msh	1025	1029	1024 w	1015 s	ring O–H bend
992 w	993 w	984 w			995			C–H and O–H bend
						937 m	935 s
871 w	872 m	871 m		870	876	888 w	888 s	C4–C5 ring str
820 m	822 s	823 m	820 s	821	825	829 m	832 vs	C–C ring str
742 vw	742 w	742 sh	710 m		745			OH oopl def
706 m br	711 m				713	758 vw	757 m	OH oopl def/ring def
	694 w	693 m	696 m
629 s	629 s	621 s	629 s	628	631	718 w	719 m	OH oopl def/ring def
588 w	589 s	581 w	588 m	588	590	704 w	707 sbr	OH oopl def/ring def
566 m	567 s	564 m	566 s	566	570	653 m	653 vs 604 vw	OH oopl def/C–C ring str
492 vw	490 w					588 w	585 m
476 vw	477 m	468 w			478	497 vw	496 w	C–O ipl def/ring def
						427 vw	427 m
448 w	448 s	452 w	448		451	385 vw	383 vw	O–H wag C–O ipl def
362 w	363 m		363		365	366 vw	369 vw	O–H wag
343 w	345 m		344		348	339 vw	338 s	O–H wag
						304 vw	304 m
295 vw	295 m		296		297	267 vw	266 m	C–O bend
271 vw	273 w		256		267	225 vw	227 w	O wag (ring)
257 w	259 m				256	190 vw	194 vw
224 w	223 m		223		224	175 vw	177 vw	C–OH bend
210 w	208 m				209	160 vw	163 vw	Lattice
181 w	180 w				178			Lattice
163 w	163 m				164			Lattice
150 vw	148 m				98	140 m	141 m	Lattice
140 vw	138 m				87	116 w	116 m	Lattice
124 m	122 w				77		103 vw	Lattice
114 w	113 w							Lattice
	91 m						94 s	Lattice
	81 s							Lattice
	73 ssp				67		75 wbr	Lattice
	43 vw						58 w	Lattice

Abbreviations: v, very; w, weak; m, medium; s, strong; sh, shoulder; sp, sharp; br, broad; bend, bending; wag, wagging; def, deformation; str, stretching; sci, scissoring; ipl, in plane; oopl, out-of-plane, respectively.

Figure 16. Measured and calculated Raman spectra for ascorbic acid. Top: AH₂ in concentrated aqueous solution, measured with 488 and 532 nm laser lines. The spectrum of water is included for reference (532 nm). Middle: AH₂ powder measured with excitation wavelength 532 nm. Bottom: Calculated spectrum for the most stable conformer (see Figure 4) as found by Gaussian modeling within a PCM water model. Assignments are given in Table 7.

Table 7 Measured Raman spectral bands (in cm⁻¹) and assignments for aqueous solutions of ascorbic acid and sodium ascorbates, compared to literature values (39, 44, 46, 48). Some data can be found in Jehlička et al. (47). Water bands are not included. For abbreviations, see Table 6

Ascorbic acid solution		Sodium ascorbate solution		Disodium di-ascorbate solution
This work Figure 16 Figure 21	Hvoslef and Klæboe (39)	This work Figure 19 Figure 21	Hvoslef and Klæboe (39)	This work Figure 21 Figure 26	Assignments
2945 m	2944 mbr			2922 s br	C–H str
2904 sh	2904 msp	2939 s	2933 s		C–H str
	2863 vw	2895 br sh	2905 s		C–H str
1760 w	1762 mbr	1718 m	1717 vs	1701 w	C˭C + C˭O str A
1692 vs	1693 vsbr	1591 s	1594 vsbr	1556 s	C˭C + C˭O str B
1650 br sh	1503 w	1436 vw	1434 vw?		C˭C str
1470 w	1470 m	1472 w	1360 mbr	1470 vw	CH2 sci
1353 w, pol	1355 w		1325 w	1343 w br	CH2 bend
1299 w	1299 m	1292 w	1295 s 1280 s	1292 w sp	ring def, CH2 wag, C–O–H bend C ?
1277 vw	1273 vw		1242 m	1228 vw w
1220 vw sh	1218 mbr	1143 w	1142 m		C–O–H bend (twi)
		1113 w	1114 m	1117 w	C–C(–O)–O str
1149 m	1150 m			1151 m sp	C–O–C str, ring def
1119 vw	1119 w		1086 vw	1116 vw
1085 vw	1087 vw		1065 w	1070 vw sh	C–O–C str, C–O–H bend
1051 m, pol	1052 m	1041 m	1043 s	1043 w	C–O–C str
1020 w	1022 w	1020 w	1018 m	1021 vw sh	ring O–H bend
979 w	981 w		975 wbr	981 vw	C–H and O–H bend
935 w	937 mbr	929 m	931 s	937 vw
875 w br	873 mbr	869 w	869 m	875 vw	C4–C5 ring str
825 s, pol	826 vs	831 m	832 vs	825 m sp	C–C ring str
766 vw	797 vw		804 wbr		OH oopl def
730 vw	768 vw	757 vw	762 w	764 vw
695 w br, pol	697 s	724 w	725 wbr		OH oopl def/ring def
664 m	664 m	706 m	707 vs	714 vw	OH oopl def/ring def
634 m	633 s	668 w	669 s	669 w	OH oopl def/ring def
587 w	589 m	640 w	640 s	642 vw	OH oopl def/C–C ring str
564 vw, pol	565 wbr	603 w	605 vs	610 w
487 vw	488 w		490 m		C–O ipl def/ring def
443 vw	445 mbr	436 vw	437 m		O–H wag C–O ipl def
392 vw	396 wbr		400 m br	350 vw	O–H wag
306 vw	305 wbr	304 vw sh	309 vs		O–H wag

Figure 17. Dependence of Raman spectra of AH₂ solution on polarization, as obtained with 532 nm excitation on the DILOR-XY instrument. VV and VH refer to vertically (V) polarized incoming beam being analyzed with a sheet polarizer (V and H) after 90° of horizontal (H) scattering. The temperature was 24°C.

Figure 18. Raman spectra of normal and deuterated (D₂O/H₂O = ∼90%) aqueous AH₂ solutions, as obtained with 532 nm on the DILOR-XY instrument. Solutions were freshly made and recorded at 24°C within 1 h after preparation.

Figure 19. Comparison of ascorbate AH⁻ Raman spectra, obtained from an aqueous solution, from a crystalline powder sample, and as calculated.

Figure 20. Raman spectra versus titration experiments on ascorbic acid dissolved in water. Bottom: Neat solution of ascorbic acid in oxygen poor water (pH = ∼2). Middle and top: After addition of the calculated amounts of NaOH solution, NaAH, and Na₂A solutions were formed (middle, pH = ∼9, and top, pH = >12). Laser excitation: 488 nm. Spectrometer: DILOR-XY.

Figure 21. Titration experiments on ascorbic acid dissolved in water after increasing amounts of NaOH solution, done with 532 and 488 nm laser excitation and the DILOR-XY instrument. Bottom: Neat solutions of ascorbic acid in water (pH ∼2). Top: Spectra versus increasing additions of base (to pH = >12).

Figure 22. Experimental Raman spectra of ascorbic acid in aqueous solution versus excitation wavelength (pH = ∼2).

Figure 24. Experimental Raman spectra of A²⁻ in aqueous solution versus excitation wavelength (pH = >12).

Figure 25. Vibrational modes labeled A, B, and C of ascorbic acid molecules AH₂, AH⁻, and A²⁻.

Table 8 Wavenumber values (cm⁻¹) and calculated Raman intensity (int, in units of Ų/amu) of vibration bands A, B, and C for AH₂, AH⁻, and A²⁻ in water. Observed wavenumber values (Obsd.) from Table 7. For intensity abbreviations see Table 6

Mode see Figure 25	A	B	C
Approx. assignment	C1–C2–C3 (iph)–sym str	C1 = O1 ooph C2 = C3 asym str	Ring def, OH bend
AH2 (H2O)	Mode 46 1784, Int = 9 Obsd. = 1760 w	Mode 45 1699, Int = 469 Obsd. = 1692 vs	Mode 43 1433, Int = 27 Obsd. = 1299 w ?
AH^− (H2O)	Mode 44 1746, Int = 57 Obsd. = 1718 m	Mode 43 1604, Int = 89 Obsd. = 1591 s	Mode 39 1415 Int = 112 Obsd. = 1292 w?
A^2− (H2O)	Mode 42 1699, Int = 99 Obsd. = 1701 w	Mode 39 1495, Int = 273 Obsd. = 1556 s	Mode 36 1369 Int = 100 Obsd. = 1292 w sp ?

Figure 23. Experimental Raman spectra of ascorbate in aqueous solution versus excitation wavelength (pH = ∼9).

Figure 26. Raman spectra of A²⁻: Experimental results in freshly made solution compared to the DFT calculated result for ion in water solvent (PCM).

Figure 27. Typical absorption spectra as obtained during titration.

Table 9 Ultraviolet absorption data of ascorbic acid species; see Figure 27

	Band	Molar	Literature values for π → π* K band in water^a
	wavelength^a	absorptivity
Species	nm	L mol^−1 cm^−1	nm	L mol^−1 cm^−1
AH2	∼247.0	∼6,800	243.5 (11)	9,600–10,000 (11)
			243 (14)	9,560 ± 350 (14)
			248 (65)	7,189 in CH3CN (64)
			245 (66)
			240 in CH3CN (64)
AH^−	∼264.8	∼16,500	265.5 (11)	14,800 (11)
			265 (14)	14,560 ± 450 (14)
			260 (65)
A^2−	∼298.4	∼7,000
	∼212.6	∼21,000

^aDependent on concentration and protolytic dissociation (14).

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