Synthesis, spectroscopic characterization and DFT calculations of β-O-4 type lignin model compounds

Article abstract of DOI:10.1016/j.saa.2013.03.075

β-O-4 type lignin model compounds with the title of Erythro-2-(2-methoxyphenoxy)-1-(3,4,5-trimethoxyphenyl)-1,3-propanediol and Erythro-2-(2-methoxyphenoxy)-1-(4-Hydroxy-3,5-dimethoxyphenyl)-1,3-propanediol were synthesised and some modifications and impr

Full text of DOI:10.1016/j.saa.2013.03.075

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 430–436
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectroscopic characterization and DFT calculations
of b-O-4 type lignin model compounds
⇑
Fatemeh Mostaghni, Abbas Teimouri , Seyed Ahmad Mirshokraei
Chemistry Department, Payame Noor University, 19395-4697 Tehran, Islamic Republic of Iran
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
ꢀ
We propose the new and easy
reaction for synthesis of b-O-4 lignin
model compounds.
ꢀ
Two model compounds is synthesised
and characterize by IR, Mass and NMR
spectroscopy.
ꢀ
ꢀ
Density functional theory (DFT)
calculations have been carried out.
The results indicates DFT gives
satisfactory results for these
compounds.
ꢀ
These data will improve methods of
synthesis and understanding of paper
industry.
a r t i c l e i n f o
a b s t r a c t
Article history:
b-O-4 type lignin model compounds with the title of Erythro-2-(2-methoxyphenoxy)-1-(3,4,5-trime-
Received 10 November 2012
Received in revised form 9 February 2013
Accepted 17 March 2013
thoxyphenyl)-1,3-propanediol
and
Erythro-2-(2-methoxyphenoxy)-1-(4-Hydroxy-3,5-dimethoxy-
phenyl)-1,3-propanediol were synthesised and some modifications and improvements on them were
introduced. These compounds were characterized by IR, Mass and NMR spectroscopy. Density functional
theory (DFT) calculations were performed for the title compounds using the standard 6-31Gꢁ basis set. IR,
13C and 1H NMR of the title compounds were calculated at the DFT-B3LYP level of theory using the 6-31Gꢁ
basis set. In this work comparison between the experimental and the theoretical results indicates that the
DFT-B3LYP method is able to provide satisfactory results for predicting the properties of the considered
compounds.
Available online 25 March 2013
Keywords:
Lignin model compounds
b-O-4 structure
Lignin synthesis
b-O-4 linkage
Ó 2013 Elsevier B.V. All rights reserved.
Arylglycerol-b-guaiacyl ether
DFT
Introduction
syringyl and p-hydroxyphenyl propane units in lignin, respec-
tively. Softwood lignin is produced mainly from coniferyl alcohol
Lignin is one of the main cell wall components in woody plants.
It is considered to be the second most abundant biopolymer after
cellulose. Lignin has a very complex three dimensional structure
consisting of three aromatic groups, i.e. p-coumaryl alcohol, conife-
ryl alcohol and sinapyl alcohol depending on the wood type. These
precursors are called monolignols and contribute to guaiacyl,
and hardwood lignin is made mainly from coniferyl alcohol and
sinapyl alcohol. In addition to these two monolignols, p-coumaryl
alcohol contributes to lignin in herbaceous plants. These monolig-
nols are considered to be polymerized via a random dehydrogena-
tive reaction mechanism to produce the complicated
biomacromolecule lignin which is distributed nonuniformly as
the result of anatomical structure. The main interunit linkages
are called b-O-4, b–b, b-5, b-1, 5–5, 4-O-5 linkages and so on. Be-
yond the structural function, lignin plays several other important
⇑
Corresponding author. Tel.: +98 311 3521804-6; fax: +98 311 3521802.
E-mail address: a_teimouri@pnu.ac.ir (A. Teimouri).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.03.075

F. Mostaghni et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 430–436
431
biological roles in plants. Because it is much less hydrophilic than
cellulose and hemicellulose, it prevents the absorption of water by
these polysaccharides in plant cell walls and allows the efficient
transport of water in the vascular tissues. Lignin also forms an
effective barrier against attack by insects and fungi. Although lig-
nin is necessary to trees, it is undesirable in most chemical paper-
making fibers and is removed by pulping and bleaching processes
[1].
The heterogeneity of lignin is caused by variations in the poly-
mer composition, size, cross linking and functional groups. Thus
our knowledge of lignin chemical structure is less precise than
our knowledge of other natural and synthetic polymers.
In this study the title compounds were synthesised and charac-
terized by IR, Mass and NMR spectroscopy. Density functional the-
ory (DFT) calculations were performed for the title compounds
using the standard 6-31Gꢁ basis set. IR, 13C and 1H NMR of the title
compounds were calculated at the DFT-B3LYP level of theory using
the 6-31Gꢁ basis set.
Experimental
Materials
All chemicals and solvents were purchased from Aldrich and
Merck. Tetrahydrofuran was freshly distilled over sodium. Silica
gel for flash chromatography was Merck Kieselgel 60 (230–
400 mesh). Thin layer chromatography (TLC) was performed on sil-
ica gel plates (Merck, Kieselgel 60 GF254) with dichloromethane/
ethyl acetate (9/1) as eluent. Spots were made visible with UV
light.
Melting points were determined on electro thermal apparatus.
The spectral measurements were performed with a FT Infrared
Spectroscope, JASCO, FT/IR-6300 (400–4000 cmꢂ1), Japan. All
NMR spectra were recorded on Bruker DRX 500 AVANCE spectrom-
eter. deuterated dimethyl-sulphoxide and chloroform was used as
solvent and tetraethyl silane as internal reference. Mass spectra
were measured using an AEI MS 902 instrument and this equip-
ment was also used for accurate mass measurements of molecular
ions.
The high sensitivity of mechanical pulps and clear woods to-
wards sunlight exposure is a well described process. It is generally
accepted that the photo yellowing of lignocellulosics is mainly due
to the photo oxidation of lignin [2–4]. This behavior is associated to
the presence of lignin functional groups that interact with the UV
portion of sunlight in the region between 300 and 400 nm. Due
to the industrial and environmental relevance of to the formation
of processes involving lignin degradation and discoloration such
as pulping, bleaching and photoyellowing, a considerable effort
has been made towards the understanding of their mechanistic as-
pects, photo- and radiation chemical methods being particularly
useful for the study of these processes. However, because of the
complexity of the lignin, the detailed studies of the reactions in-
volved in these processes are difficult.
The structure and chemistry of lignin have been described in
terms of results from a wide range of chemical or spectroscopic
methods to construct a mosaic picture of the polymer. There are
many studies available in the literature involving degradation of
lignins to low molecular weight compounds in order to obtain
structural information. Lignin dimers and trimers which contained
most of the major inter-unit linkages have been extensively re-
ported from hydrogenolysis [5] and other methods [6–12] that
can be quite expensive and time-consuming. However, knowledge
about minor lignin inter-unit linkages is still incomplete.
The determination of the structural built-up of lignin had been a
constant challenge for more than a hundred years. One of the main
difficulties is the lack of the adequate lignin isolation methods
without having the lignin undergoing secondary reactions. The
knowledge of lignin structural motifs makes possible the study of
its reactivity by using suitable lignin model compounds (LMCs),
relatively simple molecules containing isolated functional groups
and/or typical lignin bonding patterns.
Methods
(2-Methoxyphenoxy) acetic acid (1)
(2-Methoxyphenoxy) acetic acid was prepared from guaiacol
and chloroacetic acid according to a procedure described [16].
Recrystallization of the residue from water gave product melting
at 123–124 °C (yield: 83%).
IR (KBr): 3060 (w), 3011 (w), 2913 (w), 1743 (vs), 1590 (w),
1510 (vs), 1457 (m), 1426 (s), 1250 (vs), 1230 (vs), 1130 (s),
1025 (m), 754(s). 1HNMR (CDCl3): d 3.92 (3H, s, OCH3), 4.74 (2H,
s, CH2), 6.95 (1H, m), 6.97 (1H, s), 7.06 (1H, m), 7.29 (1H, s), 10.5
(1H, s, COOH)
Z-2-(2-methoxyphenoxy)-3-(3,4,5-trimethoxyphenyl)-2-propenoic
acid (2)
The use of adequate lignin model compounds is often crucial in
lignin research. Lignin model compounds are not commercially
available and therefore have to be prepared in the laboratory. In
general only small amounts of model compounds are required
[13]. Thus the obvious criteria for choosing suitable synthetic
routes for such compounds would seem to be simplicity of prepa-
ration and ease of purification rather than high yield.
A 250 ml three-necked flask was used in the experiments. The
flask was equipped with an inlet and an outlet for nitrogen, a sep-
tum for injection of reagents. A slow stream of nitrogen was al-
lowed to pass through the reaction vessel (the flow rate was
registered by means of a silicone oil seal). Using a syringe, n-Butyl
lithium in THF (42 mmol, 24 ml 1.6 M solution) was injected in to
the solution of hexamethyldisilazane (8.28 ml, 40 mmol) in THF
The arylglycerol-b-aryl ether type of lignin has been considered
to constitute the most prominent substructure in lignin for a long
time [14]. Lignin model compounds of arylglycerol-b-aryl ethers
are therefore useful in research concerning lignin. Syntheses of
such compounds have been described by several authors [15–20].
Studies of lignin structure and reactivity have been re-energized
by the emergence of modern technologies, new analytical meth-
ods, and by exciting recent findings. Theoretical methods represent
another tool that can be used to complement other techniques, a
strategy that has found extensive applications in other branches
of chemistry.
(50 ml). To achieve a-lithiation of the carboxylic acid, the mixture
was stirred magnetically for about 0.5 h. A solution of (2-methoxy-
phenoxy) acetic acid (3.65 g, 20 mmol) in THF (25 ml) was slowly
injected into the flask. The mixture was kept at ꢂ75 °C for 0.5 h
(magnetic stirring). The stirring was continued for 1.5–2 h at 45–
50 °C and after cooling to ꢂ75 °C, a solution of the methylated
syringaldehyde (20 mmol, 3.92 g) in THF (15 ml) was slowly added
to the reaction mixture. Stirring was continued for 1 h at ꢂ75 °C,
after which the mixture was stirred at room temperature over-
night, it was cooled to 0 °C and acidified to pH = 2 with 3 M HCI.
The organic layer was separated and the aqueous layer was ex-
tracted with diethyl ether (100 + 3 ꢃ 50 ml). The combined organic
layers were extracted with 0.2 M NaOH (3 ꢃ 75 ml). The extract
was acidified with 3 M hydrochloric acid (21 ml) and extracted
with ether (250 ml + 3 ꢃ 100 ml). The extract was dried (Na2SO4)
Although there may be a tendency to consider experiment and
theory as two distinct approaches with little in common, the
boundary is becoming increasingly blurred as calculations are
extensively integrated with experiment.

432
F. Mostaghni et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 430–436
and solvents were removed by film evaporation that gave an oily
residual. The crude product was chromatographed on a silica gel,
eluents dichloromethane/ethyl acetate (9:1) and (5:1)] which gave
an essentially pure Z isomer (yield, 50%).
Quantum mechanical calculations
All calculations were carried out using the Gaussian 09 quan-
tum chemistry package and Gaussian Viewer as graphical medium.
DFT calculations were performed by using the three parameter
Becke 3LYP functional, which is a hybrid of exact (Hartree-Fock)
exchange terms and gradient-corrected exchange and correlation
terms, similar to that first suggested by Becke. The usual 6-31Gꢁ
basis set was employed in the DFT calculations. Following the stan-
dard nomenclature the latter calculation will be referred to as
B3LYP/6-31Gꢁ.
Finally, the calculated normal mode vibrational frequencies,
NMR and thermodynamic properties were also calculated with
the same method of calculations. Calculated results extracted man-
ually and by using Chemcraft software version 1.6. The computed
results have been compared with the experimental data.
1HNMR (CDCl3): d 3.929 (3H, s, OCH3), 3.948 (3H, s, OCH3),
3.961 (3H, s, OCH3), 3.969 (3H, OCH3), 6.93–7.39 (7H, m, vinyl,
H–Ar), 9.89 (1H, s, COOH). 13CNMR(CDCl3): d 56.66 (OCH3), 56.69
(OCH3), 61.35 (OCH3), 67.75 (OCH3), 107–154 (Ar), 171.75 (C@C),
173.50 (C@C), 191.70 (COOH).
Erythro-2-(2-methoxyphenoxy)-1-(3, 4, 5-trimethoxyphenyl)-1, 3-
propanediol (3a)
Acid 1 (1.2 mmol, 0.5 g) was dissolved in THF (13 ml) in a
100 ml flask and 2 M borane-dimethyl sulfide complex in THF
(5 ml) was injected into the flask with a syringe during 15 min
(nitrogen atmosphere and magnetic stirring). The reaction mixture
was kept at 35 °C for 2 h. Excess of reagents was decomposed by
the addition of (1 ml) H2O. Then H2O2 (1 ml 30% H2O2) and NaOH
3 M (3 ml) was added over a 30 min period. The reaction mixture
was stirred vigorously at 35 °C for 3 h period and transferred to a
separatory funnel with 30 ml H2O and 30 ml CHCl3. The layers
were separated and the aqueous layer extracted with 2 ꢃ 50 ml
CHCl3. The organic layers were combined and dried over Na2SO4
and the solvent was removed by film evaporation. The product
crystallized on standing. Recrystallization from ethyl acetate
yielded crystals yellow needles (yield, 75%).
Results and discussion
Procedure of easy and stereo selective synthesize of lignin mod-
el compounds 3a and 3b is shown in two steps on Scheme 1.
The products obtained according to previous studies was a mix-
ture of the two possible diastereomers erythro and threo forms.
Pure erythro and threo forms of arylglcerol b-aryl ethers could be
isolated forms of reduction products by chromatography on SiO2
followed by ion-exchange chromatography [15–20].
IR (KBr): 3445 (b), 2931 (m), 2850 (w), 1574 (s), 1516 (s), 1467
(s), 1426 (s), 1364 (s), 1333 (vs), 1267 (m), 1204 (vs), 1119 (vs),
1035 (w). 1HNMR (CDCl3): d 2.04 (3H, s, CH3CO), 2.09, (3H, s, CH3-
CO), 3.89 (3H, s, OCH3), 3.93 (6H, s, OCH3), 3.94 (3H, s, OCH3), 4.25
In this work the intermediate unsaturated acids 2a and 2b were
obtained in a pure crystalline state. We used hexamethyldisilazane
with no
a-proton, as base in condensation step. This preclude of
secondary reactions and increase the yield of the condensation
step and a diastereoisomeric control can occur using a more steri-
cally crowded enolate that create Z-isomer of propionic acids (2a,
2b). The product obtained on hydroboration of compound 2 was
identified as the erythro form of compounds. Since hydroboration
is known to proceed by syn addition.
The phenolic model Erythro-1-(4-Hydroxy-3,5-dimethoxy-
phenyl)-2-(2-methoxyphenoxy)-1,3-propanediol (3b) was syn-
thesised using the tetrahydropyran-2-yl ether of syringaldehyde
and (2-methoxyphenoxy) acetic acid. No separate deprotection
step was required since removal of the tetrahydropyran-2-yl group
occurred spontaneously during the reaction of tetrahydropyran-2-yl
(1H, dd, J = 3.6 and 11.9 Hz, Hc), 4.39 (1H, dd, J = 5.8 and 11.9 Hz,
Hc), 4.58 (1H, m, Hb), 6.05 (1H, d, J = 4.8, Ha), 6.84 (2H, s, H–Ar)
6.87–6.99 (4H, m, Ar–H). Mass (CDCl3): m/e (%); (M)+ 364 (0.4),
344(9), 239 (14.6), 195 (100), 167 (5.3), 124 (10.8), 109 (15.46).
Tetrahydropyran-2-yl ether of syringaldehyde
Syringaldehyde was derivativsed essentially according to a
method for the preparation of tetra hydro pyran-2-yl ethers of
alcohols [21]. (yield, 75%).
1HNMR(CDCl3): d 1.64 (2H, m), 1.84 (2H, m), 2.02 (2H, m), 3.95
(3H, s,OCH3), 3.98 (3H, s, OCH3), 4.24 (2H, m), 5.64(1H, d, J = 7.5),
7.15 (2H, s, Ar–H) 9.83 (1H, s).
ether of syringaldehyde with
a
-lithiated 1 (or during the work-up
Erythro-1-(4-Hydroxy-3,5-dimethoxyphenyl)-2-(2-
methoxyphenoxy)-1,3-propanediol (3b)
2
The above procedure for the synthesis of arylglycerol-b-guaia-
cyl ethers was followed using tetrahydropyran-2-yl ether of
syringaldehyde as the primary material, but instead of 3 M HCl,
the equivalent amounts of 2 M H2SO4 were used for acidification
and the reaction mixture was exposed to acid conditions for about
1 h during the workup procedure. Purification of crude product by
flash chromatography using mixtures of methylene chloride and
ethyl acetate as eluents gave essentially pure 2b (yield, 37%).
Reduction of acid 2b with boran-dimethylsulfide complex gave
an essentially pure product 3b (yield 72%).
Li [N(SiMe3)2]
O
CH2
(Li)2
O
CH2
COOH
COOH
OMe
OMe
1
CHO
O
C
COOH
H
R1
R3
OMe
R2
C
R2
R1
IR (KBr): 3424 (s), 2932 (m), 2850 (w), 1575 (s), 1518 (s), 1467
(s), 1426 (s), 1366 (s), 1333 (vs), 1267 (m), 1204 (vs), 1119 (vs),
CH2OH
2
1034 (w). 1HNMR (DMSO): d 3.624 (2H, m, H
c
), 3.76 (3H, s,
OCH3), 3.80 (6H, s, OCH3), 4.276 (1H, m, Hb),4.57 (1H, t,
J = 5.6 Hz, OH ), 4. 72 (1H, t, J = 4.8 Hz, ), 5.26 (1H, d,
J = 4.8 Hz, OH ), 6.53 (2H, s, H–Ar), 6.81-6.90 (4H, m, H–Ar),
8.306 (Ar–OH). 13CNMR (DMSO): d 55.77 (OCH3), 56.26 (OCH3),
56.26 (OCH3), 61.22 (C ), 65.58 (C ), 71.33 (Cb), 103.80 (A2),
R3
H
H
C
C
O
B
OH
c
Ha
MeO
1) BH3.S(CH3)
2
2) H2O2 / NaOH
a
A
R3
3
(a) R1=R2=R3=OMe
(b) R1=R3=OMe, R2=OH
R1
c
a
103.80 (A6), 111.77(B2), 114.70 (B5), 121.10 (B6), 121.96 (B1),
132.09 (A1), 134.11 (A4), 147.10 (A3), 147.10 (A5), 148.01 (B4),
149.69 (B3).
R2
Scheme 1. Stereo selective synthesize of lignin model compounds 3a and 3b.

F. Mostaghni et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 430–436
433
Table 1
Table 3
Selected bond distances.
Comparison of the observed and calculated vibrational spectra for important signals
of compounds 3a and 3b.
Bond lengths
3a
3b
Bond lengths
3a
3b
Compound
Assignment
3a
3b
C1–C2
C2–C3
C3–C4
C4–C5
C5–C6
C6–C1
C5–O13
C6–O12
1.402
1.393
1.405
1.396
1.413
1.399
1.365
1.373
1.423
1.374
1.435
1.422
1.529
1.402
1.394
1.407
1.391
1.406
1.397
1.375
1.367
1.424
1.373
–
C9–C10
1.553
1.522
1.429
1.435
1.374
1.414
1.394
1.364
1.420
1.399
1.400
1.393
1.400
1.552
1.522
1.429
1.435
1.374
1.414
1.394
1.364
1.420
1.399
1.400
1.393
1.400
C10–C34
C34–O37
C10–O23
O23–C24
C24–C25
C24–C26
C25–O39
O39–C40
C25–C27
C27–C30
C30–C28
C28–C26
DFT
Exp.
DFT
Exp.
t
t
t
t
t
t
t
t
OH
OHph
OH
c
3718
–
3445
3445
3445
2931
2850
2850
1516
1574
1467
1426
1364
–
1333
1333
1267
1204
1119
1035
3717
3642
3679
3075
3043
2990
1576
1490
1462
1422
1368
1361
1346
1296
1288
1206
1120
1118
3424
3424
3424
2932
2850
2850
1575
1518
1467
1426
1366
1366
1333
–
a
3685
3075
2970
2924
1576
1566
1481
1422
1366
–
1347
1293
1276
1205
1121
1040
C–HAr
C–H(OMeB)
C–H(OMeA)
Ring def B
Ring def A
O
13–C14
C1–O18
12–C47
O
Asymmetric CH bending
Aromatic ring vibration B
C9–O44
C3–C9
1.423
1.530
d O–H
a, Ha
d O–Hph
d H
d H
a
a
, Hb(in plan)
, Hb(out of plan)
Ring breathing A
CAr–O
1267
1204
1119
1031
Table 2
Selected bond angles and Torsional angles.
t
Ar in plan CH bending A
Ar in plan CH bending B
Bond lengths
3a
3b
Bond lengths
3a
3b
C9–C10–O23
C3–C9–O44
108.8
113.4
112.1
108.1
108.8
113.3
112.0
121.2
118.5
118.2
108.9
113.1
111.8
108.0
108.8
113.3
112.4
122.1
118.6
118.2
C25–C27–C30
C24–C26–C28
C30–C28–C26
C4–C3–C9
C4–C5–C6
C2–C3–C9
C3–C4–C5
C25–C24–C26
C6–C1–C2
120.3
120.4
119.6
119.0
120.1
121.8
120.6
119.9
120.6
120.3
120.3
120.4
119.6
119.7
121.1
121.4
119.9
119.9
119.7
120.3
C
C
C
10–C9–O44
10–C34–O37
34–C10–O23
torsion angles in this compound listed in Table 2. The aromatic
rings are well separated and the bond distance for C3–C24 bond
is 4.534 Å and 4.588 Å in 3a and 3b, respectively.
C9–C10–C34
C3–C9–C10
C6–C1–O18
C5–O13–C14
Assignments of vibrational frequencies
C
10–O23–C24
C27–C30–C28
Dihedral angles
C3–C9–C13–C15
C3–C9–C13–O20
Since FT-IR identifies specific chemical functional groups in
polymeric compounds, it has been able to give us an estimate of
the most molecular components of a plant. For example, identifica-
tion of specific bonding patterns and their representative IR spec-
tral signals have been shown for cellulose [21,22], lignin and its
precursors [23–26]. The observed experimental and calculated
vibrational wave numbers of the optimized geometry and the pro-
posed assignments of the two compounds 1 and 2 are given in
Table 3.
77.19
76.06
O11–C9–C13–C15 –162.81 ꢂ52.49
1161.50 ꢂ162.6 O18–C15–C13–C9 ꢂ51.68
111.56 O11–C9–C13–O20 69.60
ꢂ162.91
C
21–O20–C13–C9 112.2
68.79
procedure). These compounds were characterized by IR, Mass and
NMR spectroscopy.
Molecular structure of 3a and 3b compounds
The observed and calculated spectra are relevant to each other.
Based on the comparison between the calculated and experimental
results, assignments of fundamental modes were carried out. The
assignment of the experimental frequencies is based on the ob-
served band frequencies in the infrared spectra of this species con-
firmed by establishing one to one correlation between observed
and theoretically calculated frequencies. A linearity between the
experimental and calculated wave numbers (i.e., for the whole
spectral range considered) can be estimated by plotting the calcu-
lated versus experimental wave numbers (Fig. 2). The values of the
correlation coefficients indicate good linearity between the calcu-
lated and experimental wave numbers (correlation coefficients of
0. 998).
Here DFT is validated for preliminary studies of structure and
reactivity of b-O-4 lignin model compounds. The results is shown
that molecular calculation holds great potential for studying lignin
and can provide valuable information. The optimized geometry
and structural parameters of two compounds calculated by
0
B3LYP/6-31Gꢁ level of theory. The selected bond distances (ÅA)
and bond angles (°) are listed in Tables 1 and 2 and the optimized
configurations are shown in Fig. 1.
It can be observed that the values of bond angles and other geo-
metrical parameters (bond lengths and dihedral angles) comple-
ment the experimental findings and reported results. Selected
Fig. 1. The structure and dipole moment direction of compounds 3a and 3b, optimization has been performed by the B3LYP/6-31Gꢁ method.

434
F. Mostaghni et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 430–436
4000
3500
3000
2500
2000
1500
1000
500
R² = 0.998
0
Exp
3b
Exp
3a
Fig. 2. Graphic correlation between the experimental and the theoretical frequencies of 3a, 3b obtained by B3LYP/6-31Gꢁ method.
Table 4
Table 5
Experimental and calculated 1HNMR chemical shifts (ppm) of 3a, 3b compounds.
Experimental and calculated 13C NMR chemical shifts (ppm) of 3a, 3b compounds.
Compound
Proton
3a
3b
Compound
Carbon
3a
3b
DFT
Exp.
DFT
Exp.
DFT
Exp
DFT
Exp
OHPh
HRing2
HRing1
–
–
5.5
6.8–8.2
6.6
5.3
0.3
0.7
4.1
3.8
4.3
3.9
3.8
4.1
–
8.3
6.8–6.9
6.5
4.7
5.3
4.6
3.6
3.6
4.2
3.7
3.8
3.8
–
C24
C27
C1
C5
C6
162.20
161.25
158.10
158.02
148.52
146.43
128.64
127.51
126.33
116.37
115.51
108.33
104.47
89.88
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
158.68
158.03
153.93
153.42
153.42
141.14
128.84
127.62
126.03
117.79
115.72
106.05
90.97
88.72
72.46
66.54
63.62
61.83
–
149.69
148.01
147.1
147.1
134.1
132.09
121.96
121.1
114.7
111.77
103.8
103.8
71.33
65.58
61.22
56.26
56.26
55.77
–
6.8–8.1
6.6
5.5
0.5
0.7
4.1
3.8
4.3
3.9
3.8
4.0
4.1
6.8–6.9
6.8
6.0
–
H
a
c
a
OH
OH
–
C3
Hc
4.3
4.2
4.5
3.8
3.9
3.9
3.9
C21
C23
C25
C22
C4
C2
C13
C9
C15
C42
C46
C32
C38
Hc
Hb
B OMe
A OMe
A OMe
A OMe
72.48
67.79
67.61
62.77
NMR spectra
62.41
Experimental and theoretical values for 1H, 13C NMR of two
compounds 3a and 3b are given in Tables 4 and 5.
The theoretical 1H and 13C NMR chemical shifts of 3a and 3b
have been compared with the experimental data. According to
these results, the calculated chemical shifts are in line with the
experimental findings. In order to compare the experimental
chemical shifts based on the calculations, the correlation graphics
have been presented in Fig. 3.
The carbon chemical shifts’ correlation values are found to be
0.972 for compound 3b. As shown in Fig. 3, those compounds have
18 different carbon atoms, which is consistent with the structure
based on molecular symmetry. Chemical shifts were reported in
parts per million relative to TMS for 1H and 13C NMR spectra. Spec-
tra were obtained at a base frequency of 125.76 MHz for 13C and
500.13 MHz for 1H nuclei. Relative chemical shifts were then esti-
mated by using the corresponding TMS shielding calculated in ad-
vance at the same theoretical level as the reference. The
experimental values of 1H and 13C isotropic chemical shifts for
TMS were 31.78 ppm and 201.576 ppm, respectively.
Fig. 3. Plot of the calculated versus the experimental 13C NMR chemical shifts
(ppm) of 3b compound (erythro).
Electronic structure
and coworkers [27]. The energy of HOMO and LUMO orbital’s
showed in Table 6. Fig. 4 presents the spin density isosurface for
HOMO and LUMO orbital’s of the phenolic and nonphenolic model
The experimental data suggest that the nonphenolic model is
less able to suffer photoyellowing, as proposed by Lanzalunga

F. Mostaghni et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 110 (2013) 430–436
435
Table 6
Thermodynamic properties
Theoretically computed HOMO and LUMO energies (H), ionization potential (H),
electron affinity (H), visible absorption maxima (nm), corresponding oscillator
strength (eV) for 3a and 3b compounds.
Several calculated thermodynamic parameters of two com-
pounds calculated at B3LYP levels are presented in Table 7.
Compounds
Parameter
3a
3b
HF
DFT
HF
DFT
Conclusions
HOMO energy
LUMO energy
Ionization potential (IP)
Electron affinity (EA)
Visible absorption maxima 215.08
ꢂ0.30485 ꢂ0.21404 ꢂ0.30049 ꢂ0.21353
0.06433
0.30485
ꢂ0.01831 0.06397
ꢂ0.01316
0.21353
0.01316
256.78
In previous works LDA was used as base for condensation step
that creates an alcohol impurity. Its formation might be due to
the reduction of the carbonyl function in aldehyde by a hydride
transfer from LDA present in excess. In this study we used hexam-
0.21404
0.21353
0.01316
215.16
5.7623
ꢂ0.06433 0.01831
259.76
4.7731
Corresponding oscillator
strength
5.7646
4.8284
ethyldisilazane with no
a-proton, as base in condensation step.
This preclude of this secondary reaction and increase the yield of
the condensation step and a diastereoisomeric control can occur
using a more sterically crowded enolate that create Z-isomer of
propionic acids (2a, 2b). In second step, reduction of these acids
(2a, 2b) produces erythro form of products, since hydroboration
is known to proceed by syn addition. What higher yields and also
in reduction of the amount of a contaminant invariably found in
the reaction product was resulted. However, Z- isomer of acid 2a
and 2b could be isolated in a yield of 50% and 37%.
No separate deprotection step was required since removal of
the tetrahydropyran-2-yl group occurred spontaneously during
the reaction (or during the work-up procedure).
The experimental and the theoretical investigation of the title
compounds 3a and 3b have been performed successfully by using
IR and NMR and quantum chemical calculations. This investigation
proves that the vibrational frequencies and visible spectra of pre-
pared models can be successfully predicted by HF and B3LYP meth-
ods with the used basis sets. According to calculations, the best
fittings between calculated and measured vibrational frequencies
were achieved by B3LYP theoretical level. With this level, the devi-
ations between calculated and experimental values are quite small
for a given type of vibration.
Obtained information about structures, and vibrations are a
valuable part of planning chemical experiments, as well as a sup-
port for the explanation of obtained experimental results. Thus
we can use methods are being used as part of lignin research.
We hope the combination of theoretical and experimental data
will be furnished relevant information on the properties and
behavior of lignin model compounds and help us to find the photo
oxidation mechanism and reactivity of this class of compounds and
introduced a new insight about process of photo yellowing.
Fig. 4. Spin density isosurface for HOMO and LUMO orbital’s of 3a and 3b
compounds.
(3a, 3b compounds). In HOMO 3a, the nonzero spin density (clouds
over the atoms) occurs on b–O–4 bond and the phenoxy ring and in
HOMO 3b it occurs on carbon and phenyl ring. This is in agreement
with the result which is expected for open-shell molecules, where
nonzero spin density is an indication of occurrence of unpaired
electrons [28,29]. Increase the electron density in the region of -
O-4 bond, induces it’s strength, imply a higher activation barrier
for the cleavage of the b-O-4 bond.
UV–vis spectra
The calculated visible absorption maxima of these compounds
in this study are given in Table 6. The electronic spectra of these
compounds showed an absorption band in the region 256–
259 nm this band was due to electronic transition of the aromatic
rings.
Acknowledgements
We would like to thank Fars Payame Noor University research
council for the financial support (Grant no. 1388/3/0/8/512).
Thanks are also due to supports from the Payame Noor University
in Isfahan.
Table 7
Theoretically computed energies (H), zero-point vibrational energies (kcal molꢂ1),
rotational constants (GHz), entropies (cal/mol K), and dipole moment (D) for 3a and
3b compounds.
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