Jai Bajrang Bali Ki JaiHo
AB INITIO AND DFT STUDIES ON THE
SPECTROSCOPY AND PHOTOPHYSICS OF
ANTHRANILIC AND SALICYLIC ACID
V.B. Singh
Department of Physics
Udai Pratap Autonomous College
Varanasi-221002 , India
1. INTRODUCTION
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•
•
•
The PHOTOINDUCED Excited-State Intramolecular Proton/Hydrogen atom Transfer
[ESIPT/ESIHT] process is one of the most elementary photoreactions observed in nature. To
gain experimental and theoretical understanding of the mechanism of ESIPT/ESIHT in complex
biological systems and practical applications , it is desirable to study simple systems ,
such as salicylic and anthranilic acids.
It is nowdays well established for many ESIPT systems that the proton (or hydrogen) transfer is an
extremely fast process . The femto second rise time of the red-shifted fluorescence provides a
strong argument for an essentially barrierless process in the lowest excited singlet state. The
photophysics of the ESIPT reaction has most frequently been discussed in terms of a tautomeric
form in the excited state potential energy (PE) surface due to transfer of a H-atom along a
preexciting intramolecular H-bond.
Salicylic acid (SA) has served as a model compound for the experimental and theoretical
investigation of the excited state intramolecular hydrogen - transfer . The conclusion that
hydrogen-atom transfer occurs in salicylic acid is based on the early work of Albert
Weller[1]. Weller postulated that proton transfer occurs in methyl salicylate to explain the dual
fluorescence observed in the the emission spectrum of this molecule. Weller believed that the
excited -state product was a zwitter-ion, leading to his description of this process as a proton
transfer . Although calculations indicate that the excited-state dynamics of salicylic acid involve the
motion of a neutral hydrogen atom rather than a proton.
Anthranilic acid (AA) has a similar structure to that of Salicylic acid and both are known to
form an intramolecular hydrogen bond . Anthranilic acid has this hydrogen bond between its
amino and carbonyl groups, whereas salicylic acid form intramolecular hydrogen bond between
hydroxyl and carboxylic groups. With respect to the internal rotation of the carboxyl group
two conformational isomers [ rotamers ] are expected for each of the two molecules .
1. A.Weller Z.Electrochem. 60, 1144,(1956)
SALICYLIC ACID
ANTHRANILIC ACID
SALICYLIC ACID ROTAMER
ANTHRANILIC ACID ROTAMER
SALICYLIC ACID DIMER-I
SALICYLIC ACID DIMER-II
SALICYLIC ACID DIMER-III
ANTHRANILIC ACID DIMER-I
ANTHRANILIC ACID DIMER-II
ANTHRANILIC ACID DIMER-III
Theoretical and experimental investigations on Anthranilic acid and Salicylic acid
predicted that monomer of each molecule exists under an equilibrium condition
with their corresponding cyclic dimer.It could be dimerise in different ways.
AA and SA dimmers are perhaps the smallest aromatic systems in
which both intra- and intermolecular hydrogen bonding exist and
thus constitute an ideal model to study both modes of hydrogen
transfer in a single system. Experimental information about structures of AA
and SA and their Dimers have been reported so far by electronic spectroscopy
and recently by IR spectroscopy by number of groups , however theoretical
calculations are rather limited. The key to the structure determination is
clearly to observe the intramolecular and intermolecular hydrogen
bonded OH and C=O groups and IR spectroscopy should be the most
suitable technique.
In the present work (i) The ground state IR spectra of monomers and dimmers of Anthranilic
and Salicylic acids have been studied by Ab initio and DFT calculations
to explore the both intra- and intermolecular hydrogen bonding
properties in the different conformers of the two molecules. Hydrgen
bonding properties of Salicylic Acid and Anthranilic Acid are compared.
(ii) The ab initio potential energy profile for the ground state and ground
state rotamarisation in the each molecules were derived.
(iii)The excited state energies and potential energy profiles were
determined to explore photophysical properties of these systems ,using
Time Dependent DFT and CIS calculations.
(iv) Recent Studies on the Spectroscopy and photophysics of AA and SA are
summarized and Excited-State Intramolecular Hydrogen atom
2.METHODOLOGY
• The Ground state properties of each conformer
of the Anthranilic Acid and Salicylic Acid have
been investigated using Hartee-Fock (HF) and
Density Functional Theory (DFT) methods using
different basis sets upto 6-311g(d,p) level.
The excited states have been studied using time
dependent density functional theory With B3LYP
funtional using different basis sets upto 631++g(d,p) level and configuration interaction
including single excitation (CIS) method.
All of the HF and DFT calculations reported here
were carried out using the GAUSSIAN 03 .
3.Results and Discussion
•
•
•
•
•
A : Our DFT calculation predicts two IR bands at 3585* and 3251 cm-1 corresponding to OH
stretching vibration in the ground state of the Salicylic Acid (SA). The IR band at 3585 cm-1 is clearly
assigned to the free OH stretch of the carboxylic group and the band at 3251 cm-1 is attributed to the
phenolic OH ( stretch ) , which is intramolecularly hydrogen bonded to the neighboring carbonyl group .
Our results corroborates to the experimental values obtained from the high resolution fluorescence
studies by Yahgi et al [1] and Bisht et al [2]. Thus the low frequency shift of the phenolic OH stretching
due to the intramolecular hydrogen bond is roughly estimated to be 400 cm-1 , since the free OH
stretching frequency of phenol is known to be 3657 cm-1 [ 3 ]. Our C=O stretching vibrational frequency
of SA also shows a significant low frequency shift.
On the otherhand , in the SA Rotamer , the phenolic OH stretch band was found at 3511 cm-1, while the
free carboxylic OH stretch band was predicted again at 3585 cm-1 , as for the SA . These values also
corroborates to the experimental values reported recently [1,2]. C=O stretching vibrational frequency of
SA rotamer also shows very low shift in comparison to SA.
Thse results clearly demonstrate the difference between the intramolecular hydrogen bonds in the SA and
SA Rotamer. It is well expected that the intramolecular hydrogen bond in SA is stronger than that in SA
Rotamer because the carbonyl O-atom is more basic than the carboxylic O .
We have also performed HF and DFT calculations for the Harmonic vibrational frequncies of the isomer A
of SA dimer.The IR band predicted in this conformer of SA at 3308 cm-1 is attributed to antisymmetric
combination of the phenolic OH stretches and the band predicted at 2887 cm-1 is attributed to
antisymmetric combination of the OH stretches in the (COOH)2 ring. These values also corroborates to
the experimental values . In this respect, it is noticed that phenolic OH frequency shifted by 57 cm-1 ,
upon dimer formation.
This result demonstrate that , When a strong intermolecular hydrogen bond is formed upon the
dimerization , the nonbonding electrons of the carbonyl group are strongly attracted by the carboxylic OH
group of the other unit , and it reduces the electron density between the phenolic OH and carbonyl O
atom , resulting in the decrease of the intramolecular hydrogen bond length.
Our DFT calculations for the ground state of Anthranilic Acid (AA) , predicted that NH2
Symmetric and AntiSymmetric Stretching at 3386* and 3556 cm-1. The corresponding values
for the AA Rotamer was found to be at 3452 and 3580 cm-1 respectively . The OH stretch
frequency [3592 cm-1] remains relatively unchaged after rotamerisation. These results , which
corroborates to the experimental values , demonstrate that neither of NH2 stretch vibrational
frequencies , of AA in its ground state , are strongly shifted due to intramolecular hydrogen
bond , as found in the case of SA.
So the intramolecular hydrogen bond in the AA is Weak , because the primary amines are
known to be considerably weaker hydrogen-bond doners than the hydroxyl groups.
DFT calculations for the ground state of AA Dimer-I predicts the NH2 symmetric and
Antisymmetric stretch frequencies at 3400 and 3550 cm-1 which are very close to
experimental values [4].These appear very close to the AA monomer ground state values[3386
and 3556 cm-1].
The AntiSymmetric [hydrogen-bonded ] OH stretch in the [COOH]2 ring was predicted at 2990
cm-1 , near to experimental value [5 ] . This extreme red shift is the characteristic of strong
intermolecular hydrogen bonded dimers, as found in the case of SA.
•The Calculated Results have been scaled by a factor of 0.9522.
•References:
2. T. Yahgi, A. Fuzi, T. Ebata, and N. Mikami J. Phys.Chem. A 105,10673 (2001)
3. P.B. Bisht , H. Petek , K. Yoshihara and U. Nagashima J. Phys.Chem. A 103,5290 (1995)
4. T. Watanabe, T.Ebata, S.Tanabe and N.Mikami J. Phys.Chem. A 105,408 (1996)
5. C.A. Southern, D.H. Levy, G.M. Florio, A. Longarte and T.S. Zwier
J. Phys.Chem. A 107,4032 (2003)
6. C.A.Southern, D.H. Levy, J.A.Stearns ,G.M.Florio, A.Longarte and T.S.Zwier
J. Phys.Chem. A 108, 4599 (2004)
Ground State (S0) IR of Salicylic acid at
b3lyp/6-31g** Level
1756
3415
3765
IR of ground state Salicylic acid
Rotamer , optimized at
b3lyp/6-31g**
1826
3686
3765
Experimental Ground State IR SPECRA
OF SA IN CONDENSED PHASE
1774
Ground state IR of Anthranilic acid at
B3LYP/6-31G** level
3735/71
3556
Ground state IR of anthranilic acid
Rotamer Optimized At b3lyp/6-31g**
1676.6
1806
3761
3626
Table.1 : Comparison of Calculated and Observed C=O
and OH Stretch Frequencies of SA , SA Rotamer and
SA Dimer A
SA
SA
SA
SA
SA
Rotamer Rotamer Dimer
[ Obs.]
[Calc]
[Calc.]
SA
Dimer
[Obs.]
Vibrational
Assignments
[Calc.]
[Obs.]
1668
-
1739
-
-
C=O stretching
-
-
-
-
2887
2900
OH stretching in
(COOH) ring
3251
3248
3511
3530
3308
3295
Phenolic OH
stretching
3585
3585
3585
3585
-
-
Carboxylic OH
stretching
Table.2 : Comparison of Calculated and Observed C=O
and OH Stretch Frequencies of AA , AA Rotamer and
AA Dimer A
AA
AA
AA
Dimer
[Obs.]
Vibrational
Assignments
[Obs.]
AA
AA
AA
Rotamer Rotamer Dimer
[Calc]
[ Obs.]
[Calc.]
[Calc.]
1684
-
1718
-
-
C=O stretching
3386
3394
3452
-
3400
3407
Symm NH2
stretching
3556
3542
3580
-
3550
3545
Asymm NH2
stretching
3591
3592
3585
-
2999
3000
Assymm.combinati
on of Carboxylic
OH stretch in
(COOH) ring
Rotational potential energy profile for Salicylic acid
OH
OH
O
O
H
H
O
16
14
OH
12
H
E (kcal/mol)
10
8
6
4
2
0
B3LYP/6-31G**
-2
0
Relaxed scan
50
100
150
200
250
Dihedral angle (degree)
300
350
Rotational potential energy profile for Anthranylic acid
OH
14
OH
O
O
H
12
N
H
N
H
H
10
O
E (kcal/mol)
8
OH
H
N
6
H
4
2
0
B3LYP/6-31G**
-2
0
Relaxed scan
50
100
150
200
250
Dihedral angle (degree)
300
350
20
20
18
16
15
14
E (kcal/mol)
10
10
8
6
5
4
2
0
0
Salicylic acid
Anthranilic acid
0
50
100
150
200
250
300
Salicylic acid
Anthranilic acid
-2
0
350
50
100
150
200
250
300
Dihedral angle (degree)
Dihedral angle (degree)
20
Salicylic acid
Anthranilic acid
18
16
14
E (kcal/mol)
E (kcal/mol)
12
12
10
8
Dihedral scan keeping
other parameters constatnt
6
4
2
0
0
50
100
150
200
Dihedral angle (degree)
250
300
350
350
Anthranilic acid ground state potential energy
surface
A.U
N-H bond
Distance
Energy in kcal / mol
E
Potential energy surface of anthranilic acid CIS/-31G**
Potential energy surface of
anthranilic acid at CIS/6-31G**
r [N-H] in angstrom
S0
S1
S2
S3
Table: 3 Absorption maxima (in nm units) corresponding to vertical excitation energies
of SA and SA rotamer and AA and AA rotamer
Excited
State
SA
TDDFT
CIS
CASSCF*
MRCI*
SA Rotamer
Expt.a
(B3LYP)
Oscillator
strengthb
S1
297.0
218.57 203.4
216.9
335
0.0829
S2
242.4
179.53 170.2
189.6
0.0
S3
235.2
161.41 161.4
0.1328
S4
202.1
129.83 --0.2601
T1
381.8
346.66 262.9
266.3
0.0027
T2
347.6
327.38 253.8
260.1
0.0553
T3
389.9
283.10 217.7
228.9
0.00
T4
360.77
266.58
0.1686
a
Obtained from the Ref. Yahagi et al. J.Phys.Chem.,105 ,10680 (2001)
b Obtained from TDDFT method at B3LYP/6-311++g** level
*
Values taken from the Ref Maheshwary et al [ To be published]
Excited
State
AA
TDDFT
(B3LYP)
CIS
Expt.a
TDDFT
(B3LYP)
Expt.a
Oscillator
strengthb
290.2
241.8
227.10
202.23
371.2
347.8
288.7
--
311
0.0830
0.0
0.1401
0.2600
0.0780
0.00
0.1456
--
AA Rotamer
Oscillator
strengthb
TDDFT
(B3LYP)
Expt.a
S1
323.34
235.43
349
0.1022
311.13
S2
254.47
186.75
0.0032
258.77
S3
245.77
164.09
0.00
244.85
S4
241.09
133.62
0.0332
237.75
a
Obtained from the Ref. Southern et al J.Phys.Chem.,107 ,4032 (2003)
b obtained from TDDFT method at B3LYP/6-311++g** level
Oscillator
strengthb
0.1070
0.0037
0.0002
Table: 3 Absorption maxima (in nm units) corresponding to vertical excitation energies
of and SA dimer and AA dimer
Excited
SA Dimer I
AA Dimer I
State
TDDFT
Expt.a
Oscillator
TDDFT
Expt.a
Oscillator
b
(B3LYP)
strength
(B3LYP)
strengthb
S1
304.2
332
0.191
311.12
354
0.2484
S2
302.1
0.00
307.42
0.00
S3
287.9
0.00
296.75
0.00
S4
287.3
0.0123
296.37
0.0114
a
The values obtained from the Ref. Southern et. al, J. Phys. Chem. A, 108, 4599 (2004)
The above TDDFT values are obtained at B3LYP/6-311++g** level.

Summary of Recent Experimental Theoretical studies :
The conclusion that hydrogen-atom transfer occurs in salicylic acid is based on the early work
of Albert Weller. Weller postulated that proton transfer occurs in methyl salicylate to explain the dual
fluorescence observed in the the emission spectrum of this molecule. Weller believed that the excited
-state product was a zwitter-ion, leading to his description of this process as a proton transfer .
Although calculations indicate that the excited-state dynamics of salicylic acid involve the motion of a
neutral hydrogen atom rather than a proton.
Recent experimental and theoretical results have called into question whether H-atom transfer
actually occurs in Salicylic Acid [ SA ]. Calculations by Sobolewski and Domcke [ 7,8 ] demonstrate
that the excited-state potential energy surface of AA and SA have only a single minimum , which is
closer to the enol form of AA(SA) than to Keto form.The geometry change that gives rise to the
strongly stokes-shifted emission is the result of the rearrangement of atomes in the hydrogen bonded
ring, which is termed a hydrogen-atom dislocation, rather than a full hydrogen-atom transfer.
The IR spectrum of the S1 state of SA predicts a red shift of the phenolic OH stretching vibration by
about 1400cm-1 whereas in AA a red shift of 500 cm-1 was predicted for the H-bonded NH vibration in
the S1 state [ 8].The interpretation of Bisht et al[ ] regarding the supersonic jet spectroscopy of SA
also suggests that only a partial transfer of the hydrogen atom occurs.
Recently excited-state behaviour of the hydrogen –bonded dimer of anthranilic acid studied
experimentally with combination of ultraviolet and infrared spectroscopy by Southern et al [ 6]. They
concluded that the first excited singlet state of AA Dimer has a double minimum potential in which the
electronic excitation is localised on one or other monomer in two wells.
References:
7. A.L Sobolewski and W.Domcke Chem.Phys.232,257 ,(1998)
8. A.L Sobolewski and W.Domcke J.Phys. Chem. A 108, 10917 ,(2004)
4.CONCLUSION:

1.The Calculated IR Spectra of SA and AA in the OH/NH and C=O Stretching vibrational
region clearly demonstrate the difference between the Rotamers in the each molecule and
shows that the INTRAMOLECULAR HYDROGEN BOND in SA is considerably
STRONGER than the INTRAMOLECULAR HYDROGEN BOND in AA.
These results Corroborates to the recent high level experimental observations.
 2.The HF and DFT Calculations predicted that SA and AA both are more stable than their
corresponding Rotamers. In the each molecule SA and AA Dimer-I is more stable than
Dimer II & Dimer-III ,Whereas Dimer –III is more stable than Dimer-II.
 3.Our Results provides strong support for the Reliabilty of the TD-DFT/B3LYP Method for
the calculation of Excitation energies of the excited singlet states of SA and AA , in
comparison to the other Methods[ Like CIS,CASSCF,MRCI]. The trends of Excitation
energies for the excited singlet states of the SA and AA Dimer-I are found to be same.
 4.The recent experimental and theoretical investigations indicate that Although the
Ground-state Hydrogen-Bonding properties of SA and AA [monomer] molecules differ ,
similar effect are observed in their lowest electronically excited singlet state S1. Both
molecules exhibit a dramatic change in the strength of their intramolecular Hydrogen
bonds upon electronic excitation.
The experimental and theoretical results charaterised that the H-Atom is only dislocated in
the S1 state of SA/AA monomers rather than transferred to form other tautomeric form.
However this is an erea ripe for further investigation.