density functional computations, ft-ir, ft-raman,...

Density functional Computations, FT-IR, FT-Raman, NMR and UV analysis of

3-Formyl-2-thienylboronic acid

Parveen begaum. K, Prabhu. T* , Jayasheela. K, Periandy. S, Roopakala. K, Sowbakkiyavathi. E. S

1,2 Department of physics, A.V.C. College (autonomous), Mayladuthurai, Tamilnadu, India.

3,4,5,6 Department of physics, Kanchi mamunivar center for post graduate studies (autonomous), Puducherry, India.

*Corresponding Author E-mail: [email protected]

Abstract

3-Formyl-2-thienylboronic acid was investigated by spectral and quantum

chemical computational methods. The solid phase FT-IR and FT-Raman spectra were

recorded in the region 4000-400 cm-1

and 3500-50 cm-1

respectively. The conformation,

molecular geometry and vibrational frequencies of title molecule have been calculated

by using the density functional method B3LYP with 6-311++G (d,p) basis set. The13

C

NMR and 1H NMR were calculated by using the gauge independent atomic orbital

(GIAO) method in combination with B3LYP functional and the 6-311++G (d,p) basis

set and the results were analysed in comparison with recorded experimental spectra. A

study on the electronic and optical properties; UV absorption wavelengths, excitation

energy, dipole moment, polarizability and hyper polarizability were also made using

NBO and HOMO-LUMO methods. The possible electronic transitions, donor and

acceptor orbitals were predicted by NBO method. The thermo dynamical parameters

and molecular electrostatic potential mapping were predicted theoretically and

discussed.

Key words: FT-IR, FT-Raman, NMR, UV analysis, B3LYP, molecular docking

ADALYA JOURNAL

Volume 9, Issue 1, January 2020

ISSN NO: 1301-2746

http://adalyajournal.com/199

Introduction

The molecule 3-Formyl-2-thienylboromic acid comes under Boronic acid group whose

distinctive electronic and chemical properties made this class of compounds pertinent for

application in a variety of biomedical field. As they possess a vacant p-orbital they behave as

organic Lewis acids. Under physiological conditions boronic acids effortlessly adapt to anionic

tetrahedral structure (sp3 boron) from neutral and trigonal planar structure (sp2 boron). Broad

reactivity profile, stability and lack of apparent toxicity makes boronic acids a predominantly

fascinating class of synthetic intermediates. Low toxicity and eventual degradation into the

environment friendly boric acid, boronic acids can be viewed as ‘‘green’’ compounds [1]. A wide

variety of boronic acid derivatives of divergent biologically important compounds have been

synthesized as anti-metabolites for a possible two-pronged attack on cancer [2]. In addition to

inhibition of tumor growth, the use of boron for neutron capture therapy would be possible owing

to the preferential localization of boron compounds in tumor tissues. Boronic acid analogs have

been synthesized as transition state analogs for acyl transfer reactions [3]. Boronic acid and its

derivatives have been investigated by several authors [ 1- 4]. Molecular structure of phenyl

boronic acid has been investigated by Rettig and Trotte [4]. However, the molecule 3-Formyl-2-

thienyl boronic acid has not been subjected to the complete quantum computation analysis

supported by experimental spectral data, hence the present study has been undertaken to do the

complete vibrational, structural, NMR and UV analysis of the title molecule.

Experimental studies

The titled compound is purchased from Sigma–Aldrich Chemicals which is of

spectroscopic grade and hence used for recording the spectra as such. The FT-IR spectrum of

the above compound is recorded in Bruker IFS 66V spectrometer in the range of 4000–400

cm−1

. The spectral resolution is ±2 cm−1

. The FT-Raman spectrum of the above compound is

also recorded in the same instrument with FRA 106 Raman module equipped with Nd:YAG

laser source operating at 1.064 μm line widths with 200 mW power. The spectra are recorded

in the range of 3500-50 cm-1

with scanning speed of 30 cm−1

min−1

and spectral width 2 cm−1

.

The frequencies of all sharp bands are accurate to ±1 cm−1

.

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Theoretical studies

All quantum calculations of 3-Formyl-2-thienyl boronic acid were performed with

Gaussian – 09W program package [5] on Pentium IV processor personal. The optimized

geometrical parameters of the title molecule were determined using density functional theory

(DFT) by B3LYP methods and 6-311++G(d,p) basis sets. The vibrational frequencies of

Formyl-2-thienylboromic acid were calculated with the same functional and two basis sets 6-

31++G(d,p) and 6-311++G(d,p) . In order to improve the agreement between the calculated

frequencies and the experimental frequencies, the calculated frequencies were scaled down as

suggested in literatures and the scaling factors were reported. The charge distribution of the

molecule were computed with 6-311++G(d,p) basis set. The electronic absorption spectra of

the compound was simulated with B3LYP/ 6-311++G(d,p) level in gas phase and solvent

(DMSO and ethanol) phases. Natural Bond Orbital (NBO) analysis was carried out using

NBO version 3.1. The NMR chemical shifts of the compound were also calculated using the

Gauge Independent Atomic Orbital's (GIAO) method along with B3LYP/6-311++G (d, p)

combination. The energy distribution from HOMO to LUMO, Mullikan charges and dipole

moment of the title molecule are also computed using B3LYP method with same basis set.

Result and discussion

Geometrical Analysis

The geometrical structure of the molecule along with the numbering of atoms of title

molecule is shown in Fig. 1. The global minimum energy obtained by DFT method with

functional B3LYP and basis set 6-311++G(d,p) is -841.2 Hartree. The optimized geometrical

parameters are presented in Table 1.

Fig. 1 Molecular geometry of 3-Formyl-2-thienyl boronic acid

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As shown in the table, the CC bond lengths in the five membered thiophene ring

vary from 1.37 – 1.42 Å; C1-C2 (1.39 Å) , C2-C3 (1.42 Å), C3-C4 (1.36 Å) , in which 1.39 Å

and 1.36 Å are closer to benzene CC value 1.38 Å, that shows there is a kind of conjugation

even within thiophene ring but differs slightly from that of benzene. C2-C3 (1.42 Å) value is

close to pure single bonded CC value which shows the conjugation at this point is disturbed

by CO group. The bond length C2-C14 is 1,33Å which is purely a double bonded CC value

which implies whose electronic charge density is altered by the CO group. The literature

value for C-S bond length is 1.70 Å [7-8], in this molecule there are two CS bonds, both are

1.72 Å that shows there is slight rearrangement of electronic charge distribution due to the

presence of them within the ring.

The expected C-Br bond length value is1.6 Å, in present compound C1-B8 is found to

be 1.573 Å, this is slightly less than the expected value which may be due to the withdrawal

of charges from this bond to adjacent B-O bonds. The bond length of two B-O bonds in this

molecule are 1.36 Å and 1.355 Å, which shows there is asymmetrical charge distribution

among these bonds which may be due to the uneven influence of S atom structurally. This is

also reflected in the two OH length values; the values of O9-H11 and O10-H12 are found to

be 0.96 and 0.97 Å respectively. All the CH in the ring structure is expected to be of the

length 1.08 Å [8]. In the present compound, CH bonds are having the bond length values

between 1.087-1.09 Å which shows the variation in the conjugation in thiophene ring.

The bond angle around each carbon atom is expected to be 120o

[10] due to SP2

hybridisation. But, in this molecule, only the bond angles C3-C2-C14 and S5-C4-H7 are

found to be 120o

as expected, but the other bond angles are varying between 93 o

-132 o

,

which means the bond angle are largely varied or hybridisations are drastically changed due

to the influence of both S and Br atoms.

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Table 1.

Optimized Geometrical parameter for 3-Formyl-2-thienylboromic acid Computed at B3LYP/6-

311++G(d,p).

Mulliken & Natural Charge analysis

The atomic charge analysis plays a substantial role in the quantum chemical

understanding of molecules because the atomic charges influence the properties of the

molecular systems, such as its dipole moment, bond strength, vibrational frequencies,

electronic transitions, chemical shifts and molecular polarizability etc. The entire compound

Bond

Length

(Å)

B3LYP/

6-311++G

(d,p)

Bond

Angle

( )

B3LYP/

6-311++G

(d,p)

C1-C2 1.3988 C2-C1-S5 109.0133

C1-S5 1.7295 C2-C1-B8 132.6805

C1-B8 1.5733 S5-C1-B8 118.3062

C2-C3 1.4285 C1-C2-C3 113.9138

C2-C14 1.3366 C1-C2-C14 125.6234

C3-C4 1.363 C3-C2-C14 120.4628

C3-H6 1.0814 C2-C3-C4 112.3568

C4-S5 1.7283 C2-C3-H6 123.0724

C4-H7 1.0795 C4-C3-H6 124.5708

B8-O9 1.3683 C3-C4-S5 111.4347

B8-O10 1.3552 C3-C4-H7 128.3184

O9-H11 0.9634 S5-C4-H7 120.247

O10-H12 0.9711 C1-S5-C4 93.2813

H12-O13 1.852 C1-B8-O9 115.6472

O13-C14 1.1938 C1-B8-O10 125.0784

O9-B8-O10 119.2744

B8-O9-H11 112.3468

B8-O10-H12 115.1534

C2-C14-O13 131.6987

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is stabilized by electrostatic forces due to the distribution of the charges on these atoms, thus

any change in electro negativity of an atom can cause wide change in the overall distribution

of charges in each atom and thus the properties of the entire molecular system. The atomic

charges were calculated by two methods for comparison purpose; Mulliken Population

analysis (MPA) and Natural atomic charges (NAC) methods. Both Mulliken and Natural

atomic charges of the titled compound were computed by B3LYP/6-311++G(d,p) method

and the values are presented in the Table 2, the same is also shown graphically in Fig 2 for

comparison.

Carbon atoms in the thiophene rings are expected to be equally negative. In present

study, NAC shows all C atoms with negative charge, but only C1 and C4 are in the range of

benzene carbon. C2 and C3 values are very less, this may be due to the influence of CO

attached with C2. But in MPA, only C1 (-0.006) is slightly negative and all other Carbon C2

(0.067), C3 (0.04) and C4 (0.017) atoms are positive, in which the withdrawal of charges are

expected from C to S atom and CO groups. But charges were found to be withdrawn from S

and B atoms in both the methods, hence C atoms which are attached these atoms B and S can

only be negative, as predicted by NAC.

Other hand C14 (0.599) is having highly positive in MPA and slightly negative in

NAC, this is the C atom which is directly bonded to O atom; hence it should be positive as

predicted by MPA rather than by NAC. All hydrogen atoms are either slightly negative or

positive in MPA, but highly positive in NAC. The NAC prediction in this case is reasonable

as the H atoms can only lose electrons to the C atoms to which they are attached.

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Table 2:

Mulliken Population & Natural atomic Charge for 3-Formyl-2-thienylboronic acid

Computed at B3LYP/6-311++G(d,p).

ATOM

MULLIKEN

POPULATION CHARGE

B3LYP/6-311++G(d,p)

NATURAL ATOMIC

CHARGE

B3LYP/6-311++G(d,p)

1 C -0.00672 -0.243

2 C 0.06760 -0.1548

3 C 0.04651 -0.1027

4 C 0.01765 -0.1924

5 S 0.00514 0.2638

6 H -0.00028 0.1149

7 H 0.00083 0.1159

8 B 0.00039 0.5336

9 O -0.0002 -0.4333

10 O 0.0060 -0.4398

11 H -0.0006 0.2438

12 H 0.0167 0.2506

13 O 0.2478 -0.3818

14 C 0.5991 -0.0750

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

1 C 2 C 3 C 4 C 5 S 6 H 7 H 8 B 9 O 10O

11H

12H

13O

14C

Mulliken charge

Natural charge

Fig.2. Mulliken and Natural Charge for 3-Formyl-2-thienylboronic acid

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NMR ANALYSIS:

Chemical shift calculations are fast, accurate and applicable for complex systems. The

chemical shifts for 1H and

13C atoms of the titled compound were computed for optimized

structure, supported by GIAO method. The computed chemical shift values in gas and solvent

phases are presented in Table 3 and graphical representations of the values are shown in

Figure 3 and 4.

The titled compound showed the chemical shifts of carbon atoms in thiophene rings

lies in the range 136 to 153 ppm. The same chemical shift values for aromatic ring carbon

atoms are expected between 120 - 130 ppm [8]. This shows that the conjugation is

appreciably altered by the presence of S atom in the ring. C1 and C4 which are attached

directly to S have the maximum shift in the ring 141 and 153 respectively. This shows that

these two carbon atoms draw electronic charges from S and thus their nuclear shielding have

been increased considerably. C2 has the least shift in the ring 132 ppm, this confirms a fact

that CO has captured some of the electronic charges from C2 due to the high electro

negativity of the O atom. These entire four carbon shift within the ring agrees well with the

charge prediction by NAC method rather than the MPA method. The C14 atom which is

directly attached with O atom in the CO group has shown the maximum shift 284 ppm. This

can be only due to the over deshielding i.e. over withdrawal of the electrons from this carbon

atom C14 by the attached O atom. This is completely in agreement with high positive charge

of MPA method, the prediction of slightly negative charge by NCA method is not found

suitable here.

The 1H NMR spectra interpretation is very significant when attempt is made to

measure the possible effects of highly electro negative atoms on protons [10]. The usual

scale, for PMR (Proton Nuclear Magnetic resonance) studies is about 7 to 8 ppm in aromatic

ring, between 2 to 3 for aliphatic chain. In the present study, all the H NMR chemical shift

values are in good agreement with expected range 7 to 8 ppm for aromatic ring. Thus, it is

clear the conjugation in thiophene ring is also very close to benzene ring as structural and

atomic charge analysis. But 11H and 12H which are attached to O atoms directly in boronic

acid group shows chemical shift 3.9 and 3.6 ppm respectively. Actually these hydrogen atoms

are in the aliphatic chain, hence they are expected in the range 1to 2 ppm, since they are

attached with O, their values have been enhanced due to de shielding.

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Table. 3.

Calculated 13

C NMR &1H Chemical Shifts (ppm) for 3-Formyl-2-thienylboronic acid

Computed at B3LYP/6-311++G(2d,p) GIAO.

Atom

Gas

CDCl3 Atom

Gas

CDCl3

13

Carbon 1Hydrogen

1C 152.86 153.216 6H 7.812 7.9501

2C 136.09 136.027 7H 7.563 7.7946

3C 141.068 141.436 11H 3.642 3.9664

4C 141.527 143.146 12H 3.421 3.625

14C 285.284 284.575

Fig.4. Theoretical H NMR spectra for 3-Formyl-2-thienylboronic acid

Fig.3.Theoretical C NMR spectra for 3-Formyl-2-thienylboronic acid

4 5 6 7 8

0.0

0.5

1.0

de

ge

na

racy

chemical shift

120 140 160 180 200 220 240 260 280 300

0.0

0.5

1.0

de

ge

na

rary

chemical shift

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VIBRATIONAL ANALYSIS:

The titled molecule under investigation has 14 number of atoms and thus 36 normal

modes of fundamental vibrations. Vibrational wave numbers for all the fundamental modes

of the titled compound are computed using B3LYP functional and 6-311++G (d,p)basis set

and the values along with the experimental data are presented in Table 4. The FT-IR spectra

& FT Raman of the titled compound are shown in Figure 5.

OH Vibrations

There is double OH in the boronic acid group, the stretching vibration due to these

bands is expected in the range 3700 – 3300 cm-1

[11]. The OH group vibration are likely to be

the most sensitive to the environment, so they show pronounced shifts in the spectra of the

hydrogen bonded species. The bands corresponding to OH stretching in this molecule is

observed at 3330 and in FT IR and 3300 in FT Raman respectively, theoretically these values

are obtained at much higher values. The presence of the peak at the higher end implies there

will be intermolecular Hydrogen bonding due to these OH group, however in this molecule in

the experimental spectra the peaks are only around 3300 cm-1

which means there won’t be

hydrogen bonding with this molecule due to this OH group.

OH in-plane bending band is expected at 1451cm-1

[12], this is observed at 1450 cm-1

in FT-Raman spectra and at 1349 cm-1

in FT-IR spectra. Similarly, the out of plane bending

modes are expected between 710 – 517 cm-1

, this is observed at 640 and 610 cm-1

in FT-IR

and FT-Raman in the present case. This deviation is generally expected at the out of plane

bending modes, as the interaction between various modes at this lower range is very stronger,

the bands are very close to each other at these ranges.

CH Vibrations

The titled molecule has two CH stretching vibrations. These aromatic CH stretching

usually show peaks in the characteristic region 3100 –3000cm−1

[13,14]. Aliphatic C-H

stretching vibrations lie between 3000- 2900cm-1

. In this molecule, there are only two

aromatic CH bonds, whose bands are observed at 3103, 2979 in FT-IR. These vibrations

indicate they are closer to aromatic CH values, which confirms the predictions in the previous

sections; structural and chemical shift analyses.

The C-H in-plane bending mode usually occurs as strong to weak bands in the region

of 1300 to 1200 cm-1

[14]. Experimental study of the title compound show that the C-H in-

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plane bending vibrations lie at 1384 to 1208 cm-1

in FT-Raman and FT-IR spectrum

respectively. The C-H out of plane bending vibrations are expected to occur as strong to weak

intensity bands in the region of 800-600 cm-1

[15]. The recorded FT-IR spectrum of the titled

molecule showed bands at 605 and 580 cm-1

. All these bending modes are in the expected

region, though the out of plan bending modes lie at the bottom end of the expected region

which is due to the overlapping of the CO bending modes.

THIOPHENE RING VIBRATIONS

The aromatic ring CC stretching vibrations occur generally in the region 1600-1350

cm-1

. The position and the intensity of the ring stretching bands of five member rings of

hetero atoms are more sensitive than the corresponding bands of benzene [16]. In the present

study, there is one double bond CC and three single bond CC stretching modes and they are

observed at 1506 , 1399, 1374 and 1245 cm

-1 respectively. The CC double bond lies outside

ring, hence its values are found less than its expected value 1600 cm-1

. The three CC values

within the ring is also less when compared to benzene ring values, greater than 1400 cm-1

,

this is due to the weakening of the bond due to the presence of S in the ring. All these values

are in good agreement with the theoretical wave numbers. Both the bending modes are

slightly deviated from the expected range, which indicates that they are not pure like

stretching which means lot of influence of other modes (CC & CO) occurs in this region.

The C-S stretching vibration is difficult to identify as it usually appear weak in FT-IR

spectrum. The absorption of C-S bond is found in the range 1000 to 800 cm

-1 for both

aliphatic and aromatic sulfides. These CS bands are observed in the present molecule at

wave numbers 910 and 853 cm-1

, any deviation in the wave number must be due to the five

member thiophene ring structure only.

C=O VIBRATIONS

The stretching mode of C=O group is expected in the range of 1750 to 1730 cm-1

.In

the present study, the C=O stretching band is observed at 1750 cm-1

both in the FT-IR and

FT-Raman spectra as a very strong band. This value is exactly the expected value which

shows this band remain undisturbed by any influence in this molecule. The C=O in-plane

and out-of-plane bending modes are expected in the region 625 ± 70 and 540 ± 80 cm-1

respectively [17-18].The calculated wave number for the C=O in-plane bending mode of the

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title compound appeared at 670 cm-1

and C=O out-plane bending mode is at 543 cm-1

.These

observed deviation from the expected value explains the presence of high positive and

negative potential, where the C=O group is present, causing the enhanced biological activity

of the molecule.

AVC-A2-

Name Description

4000 4003500 3000 2500 2000 1500 1000 500

100

0

10

20

30

40

50

60

70

80

90

cm-1

%T

1652.19cm-1

1349.28cm-1

753.24cm-1

1437.61cm-1

1399.51cm-1

1374.37cm-1

1208.10cm-1

1506.96cm-1

721.56cm-11070.24cm-1

853.66cm-1

3332.90cm-1 1110.20cm-1

589.75cm-1

670.78cm-12979.35cm-1

3103.26cm-1

640.97cm-11015.00cm-12892.77cm-1

991.17cm-1

910.56cm-1

2519.82cm-12425.19cm-1

2362.97cm-1

2337.20cm-1

484.66cm-12186.29cm-1

1810.79cm-1

3953.58cm-1

3925.57cm-1

3799.46cm-1

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Table.4.

Experimental FT-IR, FT-Raman and Calculated DFT B3LYP/6-311++G(d,p)

levels of vibrational frequencies (cm1

) of 3-Formyl-2-thienylboronic acid

No

frequencies (cm-1

)

VEDA Observed Calculated Assignment

IR Raman Un

scaled Scaled

1 3330 3847 3712 ν OH ν OH 100

2 3300 3664 3521 ν OH ν OH 100

3 3103 3245 3118 ν CH ν CH 91

4 2979 2976 3214 3088 ν CH ν CH 91

5 1750 1750 1851 1778 ν O=C ν OC 94

6 1506 1519 1459 ν C=C + β CH ν CC 13+ β CH 44

7 1399 1449 1392 ν CC ν CC 18

8 1374 1372 1422 1366 ν OB ν OB 42

9 1349 1384 1330 ν CC+ β CH ν CC 13+ β CH 26

10 1245 1322 1270 ν CC+ ν OB ν CC 17+ ν OB 17+OH 17

11 1208 1209 1161 β OH ν CC 22+ β OH 27

12 1110 1134 1089 β CC β OH 14+ β CC 16

13 1070 1102 1059 β CC β OB 27+ β CC 23

14 1040 1039 998 β OB β OB 47

15 1015 1007 967 β OB β OB 23

16 991 964 926 ν CS ν CS 15

17 910 913 877 ν CS ν CS 45

18 853 850 840 807 β CS β CS 85

19 753 750 753 723 β CS β CS 13

20 721 714 686 β CB β CB 45

21 670 706 678 β CO β CO 56

22 640 685 658 γ OH γ OH 53

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23 610 617 592 γ OH γ OH 25

24 605 606 582 γ CH γ CH 68

25 589 580 582 559 γ CH η CCCC 15

26 543 546 524 γ CO β CCO 34

27 484 474 455 γ CC η CCCC 64

28 450 460 442 γ CB ν CB 16

29 390 374 γ CS η CSCC 20

30 326 313 γ CS β OBO 26

31 259 248 γ OB η HOBC 17

32 232 222 Β OCB β CBO 53

33 184 176 β OBO β OBO 31

34 177 170 β CCC η CCCO 25

35 109 104 β CCC β CCC 64

36 70 67 β CSB β BCS 64

-stretching, δ -in-plane bending, γ-out-of-plane bending, -scissoring, -rocking, -twisting, δring-

in-plane bending ring, γring-out-of -plane bending ring.

NBO ANALYSIS:

Natural bonding analysis (NBO) is an effective tool for determining the chemical

interactions, charge distribution and electron transfer from filled donor or bonding orbitals or

lone pair orbital to vacant acceptor or anti bonding orbitals. The density functional theory is

used to analysis these bonding and anti-bonding interactions, by means of the second-order

perturbation theory, interms of stabilization energy (E(2)

) [19]. This energy represents the

estimation of the off-diagonal NBO Fock matrix elements, which can be determined from the

following relation [20];

2(2) ( , )

ij i

j i

F i jE E q

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where qi is the donor orbital occupancy, εi and εj are the energy in the donor and acceptor

levels and F(i,j) is the off diagonal Fock matrix elements.

In this study, the charges transferring from bonding to anti-bonding levels were

analyzed. The intermolecular hyper conjugative interactions are caused by the orbital

overlapping between n and *&* orbitals. The E(2)

values indirectly indicate the probability

of transitions, accordingly for this molecule there are seven highest possible transitions,

which are listed below in the descending order for comparison. The other important

transitions in this molecule are listed in Table 5.

From this study, the top ten probable transitions in this molecule are S5→C1-C2

(12.73 KJ/Mol , n to π*), S5→C3-C4(10.35 KJ/Mol , n to π*), C1-C2 →O13-C14 (9.54

KJ/Mol , π to π*), C1-C2→ C3-C4 (8.49 KJ/Mol , π to π*), C3-C4 →C1-C2 (7.8KJ/Mol , π

to π*), C1-S5→C2-C14 (3.42 KJ/Mol , π to ζ*), O10→C1-B8 (3.02 KJ/Mol , n to π*),

O9→B8-O10 (2.79 KJ/Mol , n to ζ*), C 1 - C 2 →O 13 - C 14 (2.66 KJ /MOL ζ to π*) ,

C 4 – S5 → C3 – H 6 (2.34 KJ /MOL ζ to ζ*)

The maximum E2 value here is 12 KJ/MOL, only the n to π* transitions have values

greater than 10 KJ/MOL, and all π to π* transitions have values in the range 10 to 7

LJ/MOL. These value of E2 are very less when compared to benzene derivatives. Thus, the

maximum probable transitions in this compound take place between sulphur atom(S) to the

antibonding acceptors π* in the ring. This measures the π delocalization within the thiophene

ring. Thus, the NBO analysis also confirms the fact that the biological activity of the

molecule is primarily due to the sulphur atoms in the thiophene rings. The electronic

transitions however can also occur entirely at unexpected region which can also be verified

theoretically and experimentally through UV-visible spectroscopy, whose principle and

discussion is presented below.

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Table. 5.

Second order perturbation theory of Fock matrix in NBO basis of 3-Formyl-2-

thienylboronic acid

Donor

Typ

e of

bon

d

Occ

up

an

cy

Acceptor

Typ

e of

bon

d

Occ

up

an

cy

En

ergy E

(2)

kca

l/m

ol

En

ergy

dif

fere

nce

E(j

)-E

(i)

a.u

. P

ola

rize

d

ener

gy F

(i,j

)

a.u

.

S 5 n 0.76942 C 1 - C 2 π* 0.20031 12.73 0.26 0.073

S 5 n 0.76942 C 3 - C 4 π* 0.1355 10.35 0.26 0.069

C 1 - C 2 π 0.88554 O 13 - C 14 π* 0.0623 9.54 0.31 0.071

C 1 - C 2 π 0.88554 C 3 - C 4 π* 0.1355 8.49 0.29 0.063

C 3 - C 4 π 0.9224 C 1 - C 2 π* 0.20031 7.82 0.29 0.064

C 1 - S 5 σ 0.98487 C 2 - C 14 σ* 0.01347 3.42 1.13 0.078

O 10 n 0.98024 C 1 - B 8 σ* 0.01614 3.02 0.98 0.069

O 9 n 0.98447 B 8 - O 10 σ* 0.01017 2.79 1.04 0.068

C 1 - C 2 σ 0.98508 O 13 - C 14 π* 0.01291 2.66 0.76 0.057

C 4 - S 5 σ 0.99039 C 3 - H 6 σ* 0.00771 2.34 1.11 0.064

O 13 - C 14 π 0.99004 O 10 - H 12 σ* 0.01029 2.24 0.85 0.055

C 3 - H 6 σ 0.98762 C 4 - S 5 σ* 0.01053 2.05 0.76 0.05

O 13 - C 14 π 0.9924 C 1 - C 2 π* 0.20031 2.03 0.41 0.041

UV-Visible spectral analysis

Theoretical UV calculations were carried out in gas phase by TD-DFT method using

B3LYP/6-311++ G(d, p) functional and basis sets in order to get a deeper perception into the

likely electronic excitations, wavelengths, oscillator strengths and major H-L contributions

of various excitations of the of the titled compound and presented in Table 6. The

experimental UV-Visible spectrum is shown in Figure 5.

In case of solvent phase, the energy gap of the cited ten top transitions are 1.73, 2.89,

3.57, 3.87, 4.09, 4.20, 4.26, 4.53, 4.75 and 5.04 eV respectively and their estimated

absorption wavelengths are 716, 428, 347, 319, 302, 294, 291, 273, 260 and 245 nm

respectively. In gas phase, the same energy gaps are 1.668, 2.946, 3.625, 3.927, 4.039, 4.325,

4.402, 4.565, 4.844, 4.97 ev and the wavelength are 742, 420, 315, 306, 286, 281, 271, 255,

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249 nm respectively. The wavelength in both phases implies that the top two transitions

(742, 420) lie only in the visible region. i.e the two n to π* transitions from S atom in the ring

are purely in the visible region. The next three π to π* (347, 319, 302) takes place at

wavelength above 300 nm. The density of states (DOS) analysis shown in Fig.5 shows that

density of states are very high only in the region 200 - 300 nm. Hence, the ζ to π* transitions

which are listed in the bottom of the top ten list are going to be very prominent in this

molecule. The oscillator strength which is the measure of the intensity of the bands also

confirms this trend, they show insignificant values for all the top five transitions in the list,

only the sixth (290 nm) and ninth (260 nm) transitions in list has considerable oscillator

strength, theoretically only these transitions can appear in the spectrum. This is also proven in

the experimental Uv-Vis spectrum where prominent peaks have appeared against the

theoretically predicted values.

Fig.5. Experimental & theoretical UV-Visible spectra & DOS spectra for 3-Formyl-

2-thienylboronic acid

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Table. 6.

Calculated and theoretical UV-Visible value of the 3-Formyl-2-thienylboronic acid

by B3LYP/6-311++G(d,p) method.

Gas phase Solvent phase (Ethanol)

λ

(nm)

E

(eV)

(f) Major Contribution

Theo. Exp.

E (eV) (f) Major Contribution λ(nm)

742 1.668 0.0008 HOMO→LUMO (95%) 716 1.730 0.0010 HOMO→LUMO (95%)

420 2.946 0.0001 H-1→LUMO (47%) 428 2.891 0.0001 H-1→LUMO (48%)

341 3.625 0.0006 H-2→LUMO (50%) 347 3.572 0.0009 H-2→LUMO (51%)

315 3.927 0.0004 HOMO→L+1 (84%) 319 3.877 0.0000 H-1→L+1 (13%)

306 4.039 0.0001 H-1→L+1 (13%) 302 4.093 0.0005 H→L+1 (88%)

286 4.325 0.0618 H-1→LUMO (44%) 294 290 4.207 0.0823 H-1→LUMO (45%)

281 4.402 0.0000 H-1→L+1 (83%) 291 4.260 0.0001 H-1→L+1 (84%)

271 4.565 0.0102 H-1→L+1 (37%) 273 4.538 0.0076 H-1→L+1 (41%)

255 4.844 0.1367 H-2→LUMO (38%) 260 260 4.759 0.2142 H-2→LUMO (41%)

249 4.978 0.0016 HOMO→L+2 (74%) 245 240 5.046 0.0022 HOMO→L+2 (87%)

HOMO-LUMO Charge transfer

The highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular

orbitals (LUMO) are the important constituent of Frontier molecular orbitals (FMO). The

HOMO energy represents the ability of electron giving; LUMO represents the ability of

electron accepting [21]; and the energy gap between HOMO and LUMO determines

molecular transport properties, chemical reactivity, electrophilic index, hardness and softness

of the molecule. The HOMO and LUMO of the molecule are computed with B3LYP function

with 6-311++ G (d, p) basis set and the pictorial diagram of the same is shown in Fig.8.

The HOMO-LUMO energy gap and different reactivity descriptors of molecule in

both levels are presented in Table 8. The calculated energy of the HOMO is -0.219 eV and

that of LUMO is -0.099 eV. The energy gap between them is -0.12 eV, which shows the

possibility of flow of energy from HOMO to LUMO. The electro negativity is a measure of

attraction for electrons in a covalent bond is found to be 0.021. The global hardness is a

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measure the resistance of an atom to charge transfer and it is found to be -0.141. The global

softness describes the capacity of an atom found to be 0.162. The electrophilicity index is a

measure total energy due to the maximal electron flow between the donors and the acceptors

and it is found to be -0.303 eV.

HOMO LUMO

Table. 7

Calculated energy value of the 3-Formyl-2-thienylboronic acid

by B3LYP/6-311++G(d,p) method.

Parameters values

EHOMO (eV) -0.219

ELUMO (eV) -0.099

EHOMO-LUMO gap (eV) -0.12

Electronegativity (χ) (eV) 0.021

Chemical hardness (η)

(eV)

-0.141

Global softness (ζ) (eV) 0.162

Electrophilicity index (ω)

(eV)

-0.303

MEP Analysis

In the present study, a 3D plot of molecular electrostatic potential (MEP) map of title

molecule is illustrated in Fig.6. The MEP which is a plot of electrostatic potential mapped the

electron density on the surface of the molecule. The importance of MEP lies in the fact that it

simultaneously displays molecular size, shape as well as positive, negative and neutral

Fig.9. HOMO and LUMO structure of 3-Formyl-2-thienylboronic acid

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electrostatic potential regions in terms of colour grading. The MEP is also useful to locate

and study the reactive regions of the molecule, the region where positive charges are found in

abundance are said to be electrophilic region as they can attach the electrons and cause

reactions. Similarly the regions which are rich in negative charges are said to be

Nucleophilic, as they can attract positive charges and cause reactions between molecules. In

the majority of the MEPs, the maximum negative regions are marked red in colour, while the

positive region are blue in colour [22].

Potential increases in the order red < orange < yellow < green < blue. The color code

of these maps is in the range between -5.27a.u. (deepest red) to 5.27a.u (deepest blue) in

compound. As can be seen from the MEP map of the title molecule, only the regions over the

three hydrogen atoms are positive and over three oxygen atoms are negative. The other

regions over the carbon atoms are closer to neutral.

Fig.12.MEP for 3-Formyl-2-thienylboronic acid

Conclusion:

The structural analysis show that the bond angles are considerable changed by

the presence of S and B atoms in the molecule, by changing the hybridisation of the carbon

atoms. The Bond lengths of the CC bonds within the thianyl ring show that there is

conjugation of electrons within the ring closer to that of benzene ring. This is also confirmed

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by the vibrational analysis; it also showed that no hydrogen bonding is possible with OH

groups of the boronic acid. The NMR chemical shifts indicate very high shift for all Carbon

atoms in the molecule due to the substitutional groups. The NBO analysis and UV-Vis

transitional analysis showed that S atom causes transition in the visible region due to n- π*

transition and all transitions which happen in the UV region are due to ζ to π* transitions in

the boronic acid group of the molecule.

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