Experimental (FT-IR, FT-Raman, 1H, 13C NMR) and theoretical study of alkali metal syringates

Article abstract of DOI:10.1016/j.saa.2013.04.016

In this work the influence of lithium, sodium, potassium, rubidium and cesium on the electronic system of the syringic acid (4-hydroxy-3,5- dimethoxybenzoic acid) was studied. This paper presents spectroscopic vibrations (FT-IR, FT-Raman) and NMR (^1<

Full text of DOI:10.1016/j.saa.2013.04.016

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
1
Experimental (FT-IR, FT-Raman, H, 13C NMR) and theoretical study
of alkali metal syringates
⇑
´
Renata Swisłocka
Division of Chemistry, Bialystok University of Technology, Zamenhofa 29, 15-435 Bialystok, Poland
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
ꢀ
Influence of alkali metals on the
electronic system of the syringic was
studied.
Methods were applied: FT-IR, FT-
Raman, and NMR quantum-
mechanical calculations.
Good correlation between
experimental and calculated IR and
NMR spectra was noted.
Aromaticity indices, atomic charges,
dipole moments and energies were
calculated.
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work the influence of lithium, sodium, potassium, rubidium and cesium on the electronic system
of the syringic acid (4-hydroxy-3,5-dimethoxybenzoic acid) was studied. This paper presents spectro-
scopic vibrations (FT-IR, FT-Raman) and NMR (1H and 13C) study of the series of alkali metal syringates
from lithium to cesium syringates. Characteristic shifts of band wavenumbers and changes in band inten-
sities along the metal series were observed. Optimized geometrical structures of the studied compounds
were calculated by the B3LYP method using the 6-311++G⁄⁄ basis set. Aromaticity indices, atomic
charges, dipole moments and energies were also calculated. The theoretical wavenumbers and intensities
of IR and NMR spectra were obtained. The calculated parameters were compared to experimental char-
acteristics of studied compounds.
Received 12 February 2013
Received in revised form 27 March 2013
Accepted 1 April 2013
Available online 10 April 2013
Keywords:
Syringic acid
Alkali metal syringates
Spectroscopic studies
Structural data
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
acid they protect the body against oxidative stress. Syringic acid
is a hydroxylic acid vastly present in edible plants and fruits. In
In previous works [1–3] it was observed that the biological
activity of benzoic acid derivatives depends on their molecular
and electronic structure. Therefore, it may be possible to find com-
pounds with no specific biological activity not by a process of trial
and error, but based on strictly described dependencies between
the structure and biological activity of compounds.
In the last years growing interest of benzoic acid derivatives as
components of the diet is noticed, because these compounds have
antioxidant properties, together with polyphenols and ascorbic
medicine it is used as a sedative and local anesthetic. 4-Hydro-
xy-3,5-dimethoxybenzoic acid has been demonstrated to exert
antioxidant, antiproliferative, anitumor and antiendotoxic
properties [4,5]. Phenolic compounds, a group of secondary plant
metabolites, are important in the growth and the development of
vascular plants. They give color to fruits and flowers, and are
responsible for their sour and bitter taste. Syringic acid is one
of the compounds included in the plants such as: cinnamon, basil,
rosemary, clove, thyme, commonly used as a spice. [6]. An
abundant source of syringic and other phenolic acids are cereal
seeds, where these compounds locate mainly in the outer plies
of grains.
⇑
Tel.: +48 85 7469796; fax: +48 85 7469781.
E-mail address: r.swislocka@pb.edu.pl
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.04.016

´
R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
291
Iwan et al. investigated properties of lanthanide(III) complexes
with 4-hydroxy-3,5-dimethoxybenzoic acid. These compounds
were characterized by elemental analysis, IR spectroscopy, X-ray
diffraction patterns, solubility, and thermal studies [7]. Belkov
et al. measured FT-IR spectra of benzoic, vanillic and syringic acids
in CCl4 solutions and in microcrystals [8]. Features of the formation
of intra-and intermolecular hydrogen bonds were analyzed. Mech-
anisms of the intermolecular interactions are shown to be defined
by the character of the substituents attached to the benzene ring in
the para- and meta-positions relative to the carboxylic group [8].
Phelps and Young [9] investigated microbial metabolism of the
plant phenolic compounds, viz. ferulic and syringic acids under
anaerobic conditions. The results of this study indicated that these
compounds can be easily decomposed by methanogenic, denitrify-
ing and sulfonic bacterial strains. The effect of some metals on the
electronic system of benzoic acid derivatives was investigated and
also reported in our earlier papers [10–15].
The influence of alkali metals (lithium, sodium, potassium,
rubidium and cesium) on the electronic system of the syringic acid
was investigated. In the present work many different analytical
methods, which complement one another, were used: FT-IR, FT-Ra-
man, 1H and 13C NMR spectra and quantum mechanical calcula-
tions (geometry of the structure, atomic charges, theoretical
spectra of IR and NMR). Optimized geometrical structures of stud-
ied compounds were calculated by B3LYP method using 6-
311++G⁄⁄ basis set. Atomic charges, aromaticity indices, energies
and dipole moments were calculated. The calculated parameters
were compared to the experimental characteristics of the studied
compounds.
and synthesized alkali metal syringates are presented in Tables
and 2, respectively. Complete assignments of all bands
1
require application of both IR and Raman methods supported
by theoretical calculations [16] and literature data [8,9,17].
Bands are numbered along with the notation used by Varsányi
[18]. The calculated wavenumbers were obtained by B3LYP
method and 6-311++G⁄⁄ basis set. Experimental FT-IR and
FT-Raman spectra of syringic acid were recorded and presented
in Fig. 2.
Characteristic vibration bands of the carboxylic group are pres-
ent in the spectra of 4-hydroxy-3,5-dimethoxybenzoic acid. There
are very intense, broad stretching bands:
m
(C@O): 1699 cmÀ1
(CAOH): 1265 cmÀ1
(IR), 1698 cmÀ1 (R), 1771 cmÀ1 (theoret.),
m
Table 1
Wavenumbers (cmÀ1), intensities and assignments of bands occurring in the
experimental FT-IR, FT-Raman and theoretical FT-IR spectra of syringic acid. The
theoretical wavenumbers were calculated in B3LYP/6-311++G⁄⁄ level.
Syringic acid
IR
Calc.
Raman
Exp.
Assignment
No.a
Exp.
Theoret.
Int.IR
3374 vsb
3248 s
3776
3762
3238
3223
3193
100.79
146.30
1.73
2.86
m
m
m
m
m
m
m
m
m
m
m
m
c(OH)
(OH)ar
(CH)
(CH)
(CH)
as(CH3)
as(CH3)
s(CH3)
(C@O)
(CC)
3084 m
3030 m
2988 w
2972 m
2941 m
2845 m
1699 vs
1618 s
3084 w
3034 w
20a
20b
2.06
3140; 3137
3081; 3065
3018; 3006
1771
18.02; 20.15
27.68; 35.34
44.60; 52.82
419.15
161.29
35.13
2973 vw
2945 w
2833 vw
1698 vs
Experimental and computational methods
1645
1624
1543
8a
8b
19a
1595 sh
1522 s
1460 vs
1594 s
(CC)
(CC)
116.29
1521 w
1468 w
1444 w
1424 w
1371 m
1322 m
1261 w
1239 w
1198 s
All reagents used were of analytical grade from Sigma–Aldrich.
Lithium, sodium, potassium, rubidium and cesium syringates were
prepared by dissolving the powder of syringic acid in water solu-
tion of the appropriate metal hydroxide in a stoichiometric ratio
of 1:1. The mixed solution was left at the 70 °C until the sample
crystallized in the solid-state. Then, the remaining solvent was re-
moved by drying in convection dryer at 105 °C for 24 h. Obtained
complexes were anhydrous in the IR spectra of solid state samples
the lack of bands characterized for crystallizing water was ob-
served. Spectrum was registered after drying.
The FT-IR spectra of 4-hydroxy-3,5-dimethoxybenzoic acid and
its salts were recorded with an Equinox 55, BRUKER FT-IR spec-
trometer. Samples in the solid state were measured in KBr matrix
pellets which were obtained with hydraulic press under 739 MPa
pressure. Raman spectra of solid samples in capillary tubes were
recorded in the range 4000–400 cmÀ1 with a FT-Raman accessory
of the Perkin Elmer System 2000. The resolution of the spectrome-
ter was 1 cmÀ1. The NMR spectra of DMSO saturated solution were
recorded with a NMR AC 200 F, Bruker unit at room temperature.
TMS was used as an internal reference.
1505; 1504
1492; 1491
1454
1417
1489; 1483
1379
35.77; 42.22
9.94; 10.06
133.28
63.87
63.01; 0.90
403.56
das(CH3)
das(CH3)
m
m
ds(CH3)
m
m
1420 s
1373 vs
1321 s
1265 m
1246 s
1206 vs
(CC)
(CC)
19b
14
CA(OH)
(CH)
2
1329
1303
14.70
90.50
b(OH)
b(OH)ar
1272
1239
1211; 1206
1171
1169; 1168
1144
33.46
m
(CAO)
b(CH)
(CH3)
b(CH)
(CH3)
OA(CH3)
b(CH)
283.62
4.51; 5.23
375.52
0.37; 0.89
402.02
88.06
3
q
9a
q
m
1154 w
1116
18b
13
1177 vs
1113 vs
1040 m
908 m
864 m
841 w
804 m
770 s
1064
943
4.31
5.26
m
m
OA(CH3)
(CH)
1116 vw
1033 m
909 w
das(CH3)
918
877
865
808
810
763
665
593
776
580
26.91
17.33
4.93
23.51
26.09
8.08
74.01
6.56
51.35
37.28
m
c
c
m
(CH)
(CH)
(CH)
(CH)
7b
11
882 vw
804 m
7a
12
5
a
(CCC)
(CH)
b(C@O)
(CC)
To calculate the optimized geometrical structures of studied
c
compounds
a Density Functional Theory (DFT) in B3LYP/6-
737 w
689 s
734 vw
689 w
676 w
581 w
546 w
490 w
u
4
311++G(d,p) level was used. Theoretical calculations were per-
formed using the GAUSSIAN 09 [16] package of programs running
on a PC computer. The theoretical IR and NMR (1H and 13C) spectra
were obtained.
671 m
581 m
530 w
488 w
460 vw
c
c
u
a
u
(C@O)
(OH)
(CC)
(CCC)
(CC)
(OH)ar
16b
6b
16a
551
520
467
1.96
42.22
108.32
c
Results and discussion
a
b
c
Ref. [17,18].
s – strong; m – medium; w – weak; v – very.
The symbol ‘‘ ’’ denotes stretching vibrations. ‘‘b’’ denotes in-plane bending
’’ designates out-of-plane bending modes; ‘‘ (CCC)’’ denotes the aromatic
(CCC)’’ designates the aromatic ring in-
Vibrational spectra
m
modes. ‘‘
c
u
ring out-of-plane bending modes. and ‘‘
plane bending modes.
a
The wavenumbers, intensities and assignments of the bands
occurring in the FT-IR and FT-Raman spectra of syringic acid

Table 2
Wavenumbers (cmÀ1), intensities and assignments of bands occurring in the IR and Raman spectra of lithium, sodium, potassium, rubidium and cesium syringates. The theoretical wavenumbers were calculated in B3LYP/6-311++G⁄⁄
level.
Li syringate
Na syringate
IR
K syringate
Rb syringate
Cs syringate
IR
Assignment
No.a
IR
Raman
Int.IR
Raman
Int.IR
IR
Raman
Int.IR
IR
Raman
Exp.
Raman
Exp.
Exp.
Theoret.
Exp.
Exp.
Theoret.
Exp.
Exp.
Theoret.
Exp.
Exp.
Exp.
3495 s
3769b
3226
3224
130.65
4.07
1.55
3410 s
3772
3226
3225
124.09
6.34
1.74
3443 s
3772
3226
3225
124.98
5.44
2.53
3418 s
3447 s
m
m
m
m
m
(OH)c
(CH)
(CH)
(CH)
as(CH3)
as(CH3)
s(CH3)
ar
3074 w
3013 vw
3001 w
2963 w
3079 w
3015 w
3003 w
3090 w
2999 w
3073 w
3011 w
2974 w
2955 w
3072 w
3014 w
3073 w
3010 vw
2995 vw
2976 w
3071 w
3017 vw
3002 w
2979 vw
20a
20b
2997 w
2976 w
2970 m
3011 w
2974 w
2955 w
2845 w
3025 w
2998 w
3135
3132
3076
3062
3015
3004
1650
1624
1531
1546
1506
1504
1491
1490
1485
1484
1445
1423
21.76
24.43
31.39
37.72
50.12
57.45
61.56
19.96
482.26
112.79
40.84
33.64
10.47
8.16
3132
3129
3073
3059
3013
3003
1651
1626
1555
1539
1507
1505
1491
1490
1485
1482
1447
1397
24.25
26.99
33.28
39.88
53.04
60.95
54.12
12.96
525.11
122.41
42.30
33.86
10.71
6.95
2977 w
2939 w
2844 w
3132
3129
3073
3059
3013
3003
1651
1625
1547
1536
1507
1505
1491
1490
1485
1483
1446
1400
24.10
26.88
33.22
39.80
52.68
60.58
63.83
13.86
307.73
230.95
41.68
33.30
11.94
5.78
2977 w
2942 w
2847 w
2938 m
2839 m
2939 w
2842 w
2938 w
2839 m
2940 w
2845 w
2941 w
2843 m
2943 w
2845 w
m
m
2840 w
1661 m
1634 m
1557 vs
1524 s
1686 m
1616 m
1560 vs
1520 m
1460 m
1684 m
1613 m
1545 vs
1520 sh
1466 m
1656 w
1592 vs
1547 w
1534 w
1486 w
1647 m
1603 sh
1578 s
1508 s
1645 sh
1614 m
1545 vs
1520 m
1466 m
m
m
m
m
(CC)
(CC)
as(COO)
(CC)
8a
8b
1603 vs
1522 w
1600 vs
1592 vs
1547 w
1539 w
1485 w
1598 vs
1553 w
1518 w
1467 sh
19a
1464 m
1456 m
das(CH3)
das(CH3)
ds(CH3)
1452 m
1448 m
1450 m
1450 m
1453 m
3.54
2.97
3.84
30.87
778.15
1.90
7.83
27.58
767.19
25.80
17.80
724.53
1414 s
1422 sh
1395 vs
1315 m
1277 s
1410 w
1385 sh
1313 sh
1410 vw
1382 sh
1311 m
1418 w
1398 sh
1313 w
1270 w
1229 m
m
m
(CC)
s(COO)
19b
1395 vs
1319 m
1277 m
1240 m
1392 m
1321 m
1288 w
1247 w
1398 s
1398 vs
1312 sh
1279 m
1238 s
1385 vs
1312 sh
1283 sh
1223 vs
1398 vs
1314 sh
1277 m
1236 m
1315 m
1277 w
1237 m
ds(CH3)
m
m
1406
1338
1296
1263
1228
1210;
1201
1171;
1168
1144
1130
1069
967
364.65
32.94
89.56
77.90
257.62
12.46;
1.53
0.45;
0.76
343.24
36.25
2.67
1409
1335
1296
1259
1228
1211;
1202
1171;
1168
1145
1131
1072
962
194.71
25.70
92.50
76.92
249.63
11.90;
2.10
0.42;
0.71
347.46
40.92
2.65
1411
1336
1296
1258
1228
1211;
1202
1171;
1168
1145
1129
1072
961
371.61
33.25
96.55
77.66
245.71
11.77;
2.35
0.43;
0.71
349.20
44.30
2.53
(CC)+ bCA(OH)ar
(CH)
14
2
1238 s
1222 s
1222 m
1202 w
b(OH)ar
(CAO)
b(CH)
1209 s
1210 vw
1211 s
1210 vw
1213 s
1202 w
1203 s
1213 s
1212 w
m
3
q
(CH3)
q
(CH3)
m
OA(CH3)
b(CH)
18b
13
1184 m
1111 vs
1040 m
955 w
912 m
874 m
856 w
818 w
785 s
1185 vw
1111w
1042 m
954 m
914 w
875 vw
1184 m
1111 vs
1040 m
955 w
912 m
874 m
856 w
810 sh
785 s
1191 w
1114 m
1033 s
947 m
915 w
873 w
1190 w
1114 m
1033 s
945 m
915 w
867 w
m
m
OA(CH3)
(CH)
1119 vs
1042 m
959 w
910 m
883 m
869 sh
833 w
793 s
4.30
1121 w
1041 m
961 w
910 w
885 vw
2.88
0.39
1111 vs
1034 m
950 vw
901 m
1107 vs
1034 m
949 w
907 m
881 m
866 w
845 w
795 s
1111 w
1038 m
949 w
911 w
??
??
836 w
806 w
das(CH3)
bs(COO)
m
c
c
m
a
c
c
779
922
890
878
831
139.44
10.87
16.66
0.98
768
921
892
881
823
98.43
12.53
16.71
1.98
782
921
898
881
831
156.46
11.04
15.30
0.98
(CH)
(CH)
(CH)
(CH)
(CCC)
(CH)
7b
11
880 m
3.60
818 w
801 w
0.39
818 m
16.99
807 m
790 m
831 w
793 s
806 m
795 m
7a
12
5
762
798
2.29
45.56
763
800
1.69
42.10
769
805
1.89
40.96
755 s
759 s
754 s
755 m
689 w
756 s
754 w
685 w
750 m
679 w
546 m
754 w
681 w
750 s
749 w
677 w
s(COO)
675 m
550 w
595
0.02
677 w
671 m
546 m
596
0.06
673 m
546 w
597
564
527
443
0.05
0.01
677 m
544 w
498 w
/(CC)
4
566
555
505
0.11
3.01
7.93
550 w
517 w
491 w
565
554
490
0.04
2.60
0.67
546 w
517 w
498 w
546 m
514 w
498 w
545 m
521 w
496 w
411 vw
/(CC)
a
bas(COO)
/(CC)
c(OH)ar
16b
6b
(CCC)
494 w
413 vw
488 w
416 w
490 w
416 w
6.74
498 w
417 w
492 m
449
90.95
442
90.58
91.71
a
b
c
Ref. [8,18].
s – strong; m – medium; w – weak; v – very.
The symbol ‘‘ ’’ denotes stretching vibrations. ‘‘b’’ denotes in-plane bending modes. ‘‘
m
c
’’ designates out-of-plane bending modes; ‘‘/(CCC)’’ denotes the aromatic ring out-of-plane bending modes. and ‘‘
a
(CCC)’’ designates the
aromatic ring in-plane bending modes.

´
R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
293
Table 3
The experimental and calculated chemical shifts of protons in the 1H NMR and carbons in the 13C NMR spectra of syringic acid and alkali metal syringates, d (ppm).
Atom positiona
Syringic acid
Syringates
Li
Na
Exp.
–
K
Rb
Exp.
–
Cs
Exp.
12.61
7.22
3.80
Calc.b
Calc.c
6.04
7.30
3.84
Exp.
Calc.b
–
Calc.c
–
Calc.b
–
Calc.c
–
Exp.
–
Calc.b
–
Calc.c
–
Exp.
–
H1
H2
5.42
7.13
3.69
–
7.25
7.35
7.44
7.24
7.40
7.42
7.18
7.35
7.42
7.16
7.14
H3a,3b,5a,5b
3.73
8.32
3.68
4.04
3.81
4.21
3.74
8.34
3.67
4.01
3.80
4.18
3.76
–
3.71
4.03
3.82
4.20
3.74
8.34
3.72
8.38
H3c,5c
H4
H6
C1
C2
C3
C4
C5
C6
C7
3.80
9.19
7.22
4.10
5.61
7.28
4.25
6.09
7.37
3.73
7.25
5.37
7.44
5.75
7.46
3.74
7.24
5.25
7.40
5.67
7.44
3.76
7.18
5.26
7.35
5.68
7.42
3.74
7.16
3.72
7.14
120.52
107.01
147.57
140.34
147.57
107.01
167.39
56.09
56.09
122.39
110.25
152.57
148.06
151.21
107.62
170.04
55.14
55.62
122.30
110.61
152.29
147.60
152.19
108.23
171.99
55.76
56.22
128.83
107.03
146.88
137.89
146.88
107.03
170.50
55.77
55.77
128.33
110.04
152.34
146.24
150.42
106.59
189.01
55.06
55.25
129.27
108.80
151.93
144.74
151.54
106.69
187.83
55.67
55.80
129.66
106.89
146.84
137.14
146.84
106.89
170.48
55.79
55.79
130.39
110.11
152.06
145.06
150.32
106.76
182.80
54.92
55.04
131.49
108.52
151.56
143.74
151.79
106.57
182.58
55.56
55.62
127.10
106.68
146.93
137.79
146.93
106.68
168.29
55.78
55.78
131.55
109.40
151.97
145.06
150.20
106.07
184.03
54.97
55.07
132.07
108.15
151.71
143.84
151.44
106.18
183.28
55.60
55.64
129.89
106.60
146.74
136.82
146.74
106.60
168.37
55.71
55.71
131.85
106.58
146.68
136.25
146.68
106.58
168.57
55.68
55.68
C8
C9
a
b
c
Atom numbering in Fig. 1.
Calculations for isolated molecules.
Calculations for the molecules in the solvent (DMSO).
(IR), 1261 cmÀ1 (R), 1379 cmÀ1 (theoret.) and
m
(OH): 3374 cmÀ1
syringates is presented in Fig. 2, where the shift of particular bands
along the mentioned above series may be observed.
(IR), 3776 cmÀ1 (theoret.). Deformation in plane vibration bands
b(C@O): 737 cmÀ1 (IR), 734 cmÀ1 (R), 665 cmÀ1 (theoret) and
Peaks at 3073–2977 cmÀ1 can be assigned to the CAH stretch-
ing modes of aromatic ring. The typical four peaks that occur in
the spectra of aromatic compounds were present in the region
1686–1410 cmÀ1. In the IR and Raman spectra of studied com-
pounds these bands are located at about 1660 cmÀ1 (8a),
1620 cmÀ1 (8b), 1520 cmÀ1 (19a) and 1420 cmÀ1 (19b). Stretching
deformation out of plane vibration bands c
(C@O): 671 cmÀ1 (IR),
676 cmÀ1 (R), 776 cmÀ1 (theoret.). Moreover, in the spectra of acid
the bands of lower intensity are present, originating from deforma-
tion vibrations b(OH) located at 1206 cmÀ1 (IR), 1198 cmÀ1 (R.),
1329 cmÀ1 (theoret.) and
c
(OH) 581 cmÀ1 (IR), 581 cmÀ1 (R),
580 cmÀ1 (theoret.) were also observed. In the spectrum of the
syringic acid also bands from the aromatic ring and functional
groups of the ring (hydroxyl AOHar and methoxy AOCH3 groups)
were present, which were also visible in the spectra of salt. Com-
parison of the spectrum of acid with those of its alkali metal salts
revealed, first of all, the disappearance of stretching vibrations
band (C@O). Instead, bands related to the stretching and
vibrations bands
mas(CH3) that occur in the ranges: 2955–
2938 cmÀ1 (IR, Raman) and
ms(CH3) in the ranges: 2847–
2833 cmÀ1 (IR, Raman). The das(CH3) (in plane bending) give the
bands in the ranges: 1468–1444, 1042–1033 cmÀ1 (IR, Raman),
and ds(CH3) in the range: 1484–1312 cmÀ1 (IR). The bands of the
m
OA(CH3) vibrations are located in the range 1190–1154 cmÀ1
.
The wavenumbers of aromatic bands numbered as 8a, 8b, 7a,
7b, 11, 12 and 16b in IR as well as in Raman spectra increase in
comparison to free acid.
The correlation between calculated and experimentally ob-
tained wavenumbers in IR spectra of syringic acid and lithium, so-
dium, potassium syringates were studied and generally good
agreement was found. The correlation coefficient R for 4-hydro-
xy-3,5-dimethoxybenzoic acid spectra is 0.9962 and lithium, so-
dium and potassium salts amount to 0.9973; 0.9964; 0.9975,
respectively.
deformation vibrations of (COOÀ) groups (mas 1578–1545 cmÀ1
,
ms 1398–1385 cmÀ1, bs 959–949 cmÀ1, bas 498–488 cmÀ1) were
found in the spectra of complexes. The wavenumbers of symmetric
in-plane deformations bas(COO) decreased in the following series
Li ? Na = K ? Rb ? Cs. The values of
creased in the spectra along the series: Rb ? Cs ? K = Li ? Na
syringates and they are respectively: 451, 452, 465, 467 cmÀ1
D
b = bs(COO) À bas(COO) in-
.
The wavenumbers as well as intensities of the aromatic ring vibra-
tion bands did not significantly change. The IR spectra of metal
(a)
1
(b)
2
7
1
2
6
7
1
2
1
6
2
1
4
6
2
6
2
5b
3b
5a
3a
5b
5a
3a 3b
8
9
8
3
5
9
4
5
3
5c
5
3
3c
3
5c
5
3c
4
4
4
4
Fig. 1. Structures of: syringic acid (a) and sodium syringate (b).

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R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
294
(g)
(f)
(e)
(d)
(c)
(b)
(a)
4000
3000
2000
1000
Wavenumber (cm-1)
Fig. 2. Experimental FT-Raman spectra (a), FT-IR spectra (b) of syringic acid and the FT-IR spectra of salts: lithium (c), sodium (d), potassium (e), rubidium (f) and cesium
syringates (g).
NMR spectra
One of the most important factors that affect the chemical
Data for the last two salts were only experimentally obtained.
Almost all protons in salts were shifted diamagnetically in com-
parison to acid. This tendency suggests that introduction of alkali
metal atoms causes the decrease in the ring current intensity.
The largest changes of chemical shifts were observed at H4
atom. It indicates a decrease in the electronic charge density
around this nucleus and a decrease in the screening effect. The
differences between chemical shifts of proton atoms along metal
series are small and irregular. Theoretical an experimental sig-
nals from carbons no. 2, 3, 4, 5, 6, 8, and 9 were shifted down-
field in comparison to the appropriate signals in the spectrum of
shift is the electron density. Increasing the electron density
causes a higher field intensity and the decrease – lower field
strength. Theoretically as well as experimentally obtained 1H
NMR and 13C NMR chemical shifts of syringic acid and alkali
metal syringates are presented in Table 3. The numbering of
atoms is shown in Fig. 1. The experimental proton (1H) and
carbon (13C) NMR spectra of syringic acid are presented in
Figs. 3 and 4.

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R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
295
Fig. 3. The experimental proton (1H) NMR spectra of syringic acid.
Fig. 4. The experimental carbon (13C) NMR spectra of syringic acid.
ligand, whereas the signals from carbons no. 1 and 7 were
shifted upfield.
The linear correlation was observed between carbon NMR the-
oretical and experimental data of studied compounds. The correla-
tion coefficient (R) for 13C NMR spectra are amount to 0.999
(calculations for isolated molecules); 0,999 (calculations for the
molecules in DMSO) for acid, 0.995; 0,996 for lithium, 0,998;
0,998 for sodium and 0,997; 0,998 for potassium syringates.

´
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R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
Table 4
Table 6
Dipole moments (Debye) and total energy (Hartree,
1
hartree = 2625.5 kJ/mol)
The calculated aromaticity indices for syringic acid and syringates lithium, sodium
and potassium.
calculated using B3LYP/6-311++G⁄⁄ method.
Syringic acid
Syringates
Li
Aromaticity indices
Syringic acid
Syringates
Li
Na
K
Na
0.9734
96.40277
0.9974
0.9377
0.014
K
Dipole moment
Energy
0.314
À725.308
5.088
À732.306
8.000
À887.067
7.372
À132.469
HOMA
I6
Aj
0.9636
94.8130
0.9956
0.9158
0.018
0.9692
95.4470
0.9966
0.9273
0.016
0.9728
96.1029
0.9975
0.9399
0.014
BAC
D
(CC)a
a
Differences between the longest and the shortest bonds in the ring.
Table 5
The bond lengths and angles of the structures of syringic acid and its alkali metal salts
calculated in B3LYP/6-311++G⁄⁄ level (atom numbering on Fig. 1).
Syringic acid
Syringates
bonds in studied molecules are presented in Table 5. The atoms
numbering is shown in Fig. 1. Substitution of metal cation in a
place of carboxylic group hydrogen of syringic acid does not cause
significant changes in the electronic charge distribution of the ring.
The significant increase of C7AO1, C1AC7, O1AM, O2AM bond
lengths is observed in the series acid ? lithium ? sodium = potas-
sium. The bond lengths of C7AO2 insignificantly decrease in the
above order. In the case of angles, the increase was observed,
C1AC6AC7, C1AC7AO2, C7AO1AM, C7AO2AM but decrease was
noticed for: C1AC2AC7, C1AC7AO1, C3AO3AC8, O1AMAO2,
C7AMAO1.
Dipole moments, energies and geometric aromaticity indices,
were calculated and also shown in Tables 4 and 6. Comparing the
values of dipole moment for molecule of 4-hydroxy-3,5-dimeth-
oxybenzoic acid and its salts one can observe an increasing ten-
dency: 0.314 D for free acid; 5.088 D for lithium salt; 7.372 D for
potassium; for sodium salt 8.000 D. Values of energy decrease in
the series: K ? H ? Li ? Na. Almost all aromaticity indices in-
crease in the following order: acid ? lithium ? sodium ? potas-
sium. The aromaticity indexes calculated for benzoic acid [17]
(HOMA = 0.982; Aj = 0.998; BAC = 0.950; I6 = 96.89) are higher than
those obtained for syringic acid (HOMA = 0.964; Aj = 0.996;
BAC = 0.916; I6 = 94.81). This suggest that the syringic acid mole-
cule possesses lower aromatic properties than the benzoic acid
molecule.
Atomic charges on the atoms of 4-hydroxy-3,5-dimethoxyben-
zoic acid molecule and its alkali metal salts were calculated by
Mulliken, APT (atomic polar tensor), NBO/NPA (natural bond orbi-
tal/ natural population analysis) methods (Table 7). The electron
density around C1 atom increased for lithium and sodium, and
then decreased for potassium syringate. Growing electron density
around the metal atoms was observed in a series of acid ? lith-
ium ? sodium ? potassium. The decrease of the charge around
atoms C6, C1, O2 was observed.
The correlation between theoretical and experimental data
was studied. The correlation between atomic charges on hydro-
gen atoms (obtained by Mulliken, APT and NBO/NPA methods)
and the appropriate experimental chemical shifts (1H NMR)
Li
Calc.
Na
Calc.
K
Calc.
Calc.
Bond lengths (Å)a
C1AC2
C2AC3
C3AC4
C4AC5
C5AC6
C1AC6
C1AC7
C7AO1
C7AO2
O1AH1/M
O2AM
C2AH2
C3AO3
O3AC8
C4AO4
O4AH4
C5AO5
O5AC9
C6AH6
1.400
1.398
1.388
1.403
1.404
1.394
1.398
1.490
1.277
1.276
1.851
1.850
1.080
1.376
1.423
1.357
0.967
1.362
1.421
1.080
1.397
1.397
1.394
1.405
1.404
1.387
1.399
1.478
1.362
1.212
0.968
–
1.079
1.359
1.422
1.353
0.968
1.372
1.426
1.081
1.394
1.403
1.402
1.389
1.397
1.501
1.270
1.272
2.204
2.203
1.080
1.364
1.419
1.360
0.967
1.378
1.422
1.080
1.395
1.403
1.402
1.389
1.397
1.502
1.271
1.272
2.203
2.203
1.080
1.364
1.419
1.300
0.967
1.378
1.422
1.080
Angles (°)
C1AC2AC3
C2AC3AC4
C3AC4AC5
C4AC5AC6
C5AC6AC1
C2AC1AC6
C1AC7AO1
C1AC7AO2
O1AC7AO2
C7AO1AH1/M
C7AO2AM
C1AC2AH2
C2AC3AO3
C3AO3AC8
C3AC4AO4
C4AO4AH4
C4AC5AO5
C5AO5AC9
C5AC6AH6
120.13
119.41
119.77
120.98
118.94
120.77
113.29
125.31
121.40
106.36
–
119.12
125.10
118.25
119.67
107.37
113.19
118.46
122.27
118.90
121.09
119.66
119.37
120.32
120.67
119.72
119.83
120.45
82.96
119.02
121.17
119.50
119.46
120.44
120.43
118.45
118.53
123.01
88.00
120.44
119.46
119.50
121.17
119.02
120.43
118.53
118.45
123.01
88.03
83.05
87.03
88.00
118.68
125.72
118.28
120.57
107.12
115.54
117.99
121.66
118.51
125.66
118.20
120.60
106.97
113.19
118.21
122.47
117.86
125.04
117.92
119.91
106.97
113.17
118.20
122.47
1 Å = 10À10
m
a
were calculated. The correlation coefficients
R amount to
0.482; 0.892; 0.916, respectively (Fig. 5). From these data one
could draw following conclusion: NBO/NPA method shows the
best correlation with experimental results. The total charges
on the aromatic ring as well as on COOÀ group were calculated.
The increase of total charge of carboxylate group are observed
in the order Li < Na < K when APT method was used for calcula-
tion. Using NBO/NPA method no changes were observed in
above series. The total charge in aromatic ring is not changed
in the series of alkali metal syringates irrespective of used
method.
Corresponding values of correlation coefficient of 1H NMR were
found lower.
Calculated data
Optimized geometrical structures of syringic acid and lithium,
sodium, potassium syringates were calculated using B3LYP/6-
311++G⁄⁄ method [16]. The bond lengths and the angles between

´
R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
297
Table 7
The atomic charge (ea) calculated by B3LYP/6-311++G⁄⁄ method for syringic acid and lithium, sodium and potassium syringates.
Atom
Syringic acid
Syringates
Li
Na
K
Mulliken
ATP
NBO/NPA
Mulliken
ATP
NBO/NPA
Mulliken
ATP
NBO/NPA
Mulliken
ATP
NBO/NPA
C1
C2
C3
C4
C5
C6
C7
C8
C9
O1
O2
O3
O4
O5
H1/M
H2
H3a
H3b
H3c
H4
H5a
H5b
H5c
H6
1.481
À0.308
À0.087
0.506
0.472
0.460
À0.164
À0.255
0.287
0.277
0.265
2.480
À0.282
À0.116
0.522
0.445
0.468
À0.148
À0.256
0.284
0.266
0.261
2.650
À0.226
À0.142
0.535
0.424
0.481
À0.141
À0.262
0.283
0.258
0.260
1.796
À0.251
À0.142
0.536
0.426
0.482
À0.140
À0.261
0.283
0.258
0.260
À0.329
À0.552
À0.504
À0.456
À0.149
0.277
À0.297
À0.298
À0.207
À0.307
À0.172
À0.253
À0.267
0.299
0.210
0.155
0.155
0.195
0.304
0.166
0.166
0.182
À0.389
À0.565
À0.523
À0.449
À0.302
À0.179
À0.293
À0.299
À0.339
À0.341
À0.176
À0.262
À0.168
0.190
0.214
0.151
0.151
0.190
0.301
0.161
0.161
0.177
À0.357
À0.506
À0.652
À0.444
À0.271
À0.337
À0.294
À0.298
À0.469
À0.469
À0.177
À0.269
À0.268
0.442
0.223
0.149
0.149
0.187
0.300
0.159
0.159
0.174
À0.299
À0.598
À0.582
À0.509
À0.199
À0.086
À0.294
À0.300
À0.461
À0.466
À0.177
À0.268
À0.268
1.017
0.208
0.150
0.150
0.186
0.299
0.159
0.159
0.173
À0.101
À0.251
À0.122
À0.259
À0.147
À0.266
À0.148
À0.265
1.472
0.788
1.429
0.766
1.374
0.766
1.410
0.765
0.525
0.519
À0.206
À0.201
À0.696
À0.615
À0.526
À0.651
À0.571
0.484
0.232
0.170
0.170
0.193
0.525
0.522
À0.204
À0.200
À0.825
À0.828
À0.530
À0.659
À0.575
0.886
0.236
0.169
0.169
0.189
0.527
0.524
À0.203
À0.199
À0.824
À0.826
À0.532
À0.664
À0.577
0.917
0.235
0.168
0.168
0.187
0.527
0.523
À0.203
À0.199
À0.824
À0.826
À0.532
À0.663
À0.577
0.915
0.234
0.168
0.168
0.188
À0.775
À0.874
À0.877
À0.768
À0.890
0.304
À1.036
À1.037
À0.881
À0.766
À0.895
0.829
À1.018
À1.019
À0.885
À0.762
À0.900
0.854
À1.074
À1.075
À0.885
À0.764
À0.899
0.950
0.093
0.097
0.099
0.099
À0.034
À0.034
0.004
À0.035
À0.035
À0.002
0.331
À0.027
À0.027
À0.001
0.096
À0.037
À0.037
À0.006
0.327
À0.029
À0.029
À0.005
0.097
À0.037
À0.037
À0.006
0.328
À0.029
À0.029
À0.004
0.098
0.336
0.488
0.176
0.176
0.192
0.486
0.173
0.173
0.189
0.485
0.172
0.172
0.187
0.485
0.172
0.172
0.187
À0.023
À0.023
0.006
0.200
0.097
0.237
0.211
0.236
0.219
0.235
0.207
0.234
a
e = 1.6021892 Â 10À19C.
Fig. 5. The correlation between calculated atomic charges (Mulliken, APT, NBO/NPA methods) on hydrogen atoms and the appropriate experimental chemical shifts (1H
NMR).
parison to appropriate signals in the spectrum of acid. The chemi-
cal shifts of protons H3, H4 and H5 decrease in the series:
Li ? Na ? K ? Rb ? Cs syringates as consequence of the circular
current weakening and therefore an increase in the screening of
aromatic protons. This is an evidence that alkali metal perturb
the aromatic system of the molecule.
The highest correlation coefficients were obtained for experi-
mental and theoretical data calculated by the NBO/NPA method.
Spectral characteristic of the alkali metal syringates was per-
formed. Replacement of carboxylic group hydrogen with alkali me-
tal ions brought about some characteristic changes in molecular
spectra as well as in geometrical structure of studied molecules.
Obtained data of total energy indicated that stabilization of studied
molecules increases along the series: K ? H ? Li ? Na. The values
of dipole moment show an increasing tendency in series: syringic
acid ? K syringate. HOMA, Aj, BAC and I6 aromaticity indices show
Conclusions
The theoretical parameters were compared to the experimental
characteristics of studied compounds. Good correlation between
experimental and calculated IR and NMR spectra was noted. The
correlation coefficients (R) for IR spectra for syringic acid, lithium,
sodium, and potassium syringates amount to 0.9962; 0.9973;
0.9964; 0.9975, respectively. The corresponding values for 13C
NMR spectra are in the range 0.995 – 0.999. The calculated energy
decreases in order: K ? H ? Li ? Na. Experimental studies (FT-IR,
FT-Raman) showed that alkali metal ions affect the electronic
charge distribution in the aromatic ring. The intensity and wave-
numbers in the case of bands number as 20a, 2, 4, 16b, 19a (IR),
20b, 19b, 2, 13, 4 (R) decrease in comparison to free acid, which
shows disorder of the electron ligand. In the 1H NMR spectra of
syringates the signals from all protons are shifted upfield in com-

´
R. Swisłocka / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 111 (2013) 290–298
298
´
[10] M. Kalinowska, R. Swisłocka, W. Lewandowski, J. Mol. Struct. 834–836 (2007)
that the salts possess higher aromaticity character then the acid
molecule. Good agreement was obtained between the experimen-
tal and calculated infrared spectra.
572–580.
´
[11] R. Swisłocka, E. Regulska, M. Samsonowicz, T. Hrynaszkiewicz, W.
Lewandowski, Spectrochim. Acta A 61 (2005) 2966–2973.
[12] E. Regulska, M. Samsonowicz, R. Swisłocka, W. Lewandowski, J. Mol. Struct.
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744–747 (2005) 353–361.
[13] R. Swisłocka, M. Samsonowicz, E. Regulska, W. Lewandowski, J. Mol. Struct.
Acknowledgement
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792 (2006) 227–238.
´
[14] P. Koczon, J. Piekut, M. Borawska, W. Lewandowski, J. Mol. Struct. 651–653
This work was supported by the Polish Ministry of Science and
Higher Education in the framework of Grant No. N N312 111838.
(2003) 651–656.
[15] P. Koczon´ , P. Mos´cibroda, K. Dobrosz-Teperek, W. Lewandowski, J. Mol. Struct.
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