LIF excitation spectra of jet-cooled 3,5-dicyanoaniline

Article abstract of DOI:10.1039/b305797f

The infrared and Raman spectra for crystalline 3,5-dicyanoaniline (3,5-DCA) and the laser induced fluorescence (LIF) excitation spectra of vacuum jet cooled molecules have been investigated. The ground S₀(A ₁) and excited S₁(B₂) state geometries, charge distributions and vibrational frequencies of 3,5-DCA have been evaluated by ab initio calculations. The Dushinsky matrices have been used for the mode correlation and assignments of the transition frequencies in LIF excitation spectra of 3,5-DCA (-NH₂) and 3,5-DCA(-ND₂) as well as the IR and Raman spectra for deuterated species. It has been found that the transitions I₀² involving inversion motion of the amino group in 3,5-DCA show remarkable intensities suggesting substantial change in the potential energy function upon electronic excitation for the out-of-plane motion of the hydrogen atoms in the amino group. The barrier height for the inversion in the ground electronic state of 3,5-DCA seems to be the same order as the zero point energy. For the lowest excited electronic state the potential function for the inversion has single minimum but shows substantial anharmonicity. The intensity distribution in the LIF excitation spectrum has been modeled using multidimensional Franck-Condon factors. Geometry changes for S₀ → S₁ excitation of 3,5-DCA are discussed.

Full text of DOI:10.1039/b305797f

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LIF excitation spectra of jet-cooled 3,5-dicyanoaniliney
Przemysław Kolek,a Katarzyna Pirowska,a Łukasz Chacagab and Jan Najbar*a
a
Department of Physical Chemistry and Electrochemistry, Faculty of Chemistry,
Jagiellonian University, 3 Ingardena, 30-060 Cracow, Poland
Department of Organic Chemistry, Faculty of Chemistry, Jagiellonian University,
3 Ingardena, 30-060 Cracow, Poland. E-mail: ucnajbar@kinga.cyf.kr.edu.pl
b
Received 23rd May 2003, Accepted 12th August 2003
First published as an Advance Article on the web 27th August 2003
The infrared and Raman spectra for crystalline 3,5-dicyanoaniline (3,5-DCA) and the laser induced fluorescence
(LIF) excitation spectra of vacuum jet cooled molecules have been investigated. The ground S0(A1) and excited
S1(B2) state geometries, charge distributions and vibrational frequencies of 3,5-DCA have been evaluated by
ab initio calculations. The Dushinsky matrices have been used for the mode correlation and assignments of the
transition frequencies in LIF excitation spectra of 3,5-DCA (ꢀNH2) and 3,5-DCA(ꢀND2) as well as the IR and
Raman spectra for deuterated species. It has been found that the transitions I20 involving inversion motion of the
amino group in 3,5-DCA show remarkable intensities suggesting substantial change in the potential energy
function upon electronic excitation for the out-of-plane motion of the hydrogen atoms in the amino group. The
barrier height for the inversion in the ground electronic state of 3,5-DCA seems to be the same order as the zero
point energy. For the lowest excited electronic state the potential function for the inversion has single minimum
but shows substantial anharmonicity. The intensity distribution in the LIF excitation spectrum has been
modeled using multidimensional Franck–Condon factors. Geometry changes for S0 ! S1 excitation of
3,5-DCA are discussed.
aniline derivatives. LIF excitation spectra and dispersed fluor-
1. Introduction
escence of 3-cyano- and 4-cyanoanilines were investigated by
Phillips and coworkers.18–20 LIF excitation spectra of jet-
cooled 2-cyanoaniline and 3-cyanoaniline were recently stu-
died by Kolek et al.21 The presence of the CCN bending mode
has been identified in the LIF excitation spectra of 3-cyanoani-
Aniline and its derivatives are of fundamental interest in spec-
troscopy.1–8 They undergo substantial geometry changes
between the ground and excited electronic states. The ground
state potential energy function of aniline shows a double mini-
mum in the inversion mode.9,10 In the lowest singlet excited
state the corresponding potential function in the inversion
mode has very small barrier at planar geometry. The laser
induced fluorescence (LIF) excitation spectra of jet-cooled
aniline molecule show significant intensity of the bands related
to the inversion mode.8–10
The changes of hybridization of the nitrogen atom in the
amino group occurring due to substitution of a molecular
group or electronic excitation of aniline derivatives are impor-
tant topics in the fields of photochemistry and photobiology.
Particularly, large geometrical transformations of the excited
4-cyano-N-dimethylaniline molecule and its derivatives have
attracted much attention.12,14–16 The most important effect
involves rotation of the dimethylamino group upon electronic
excitation of 4-cyano-N-dimethylaniline. Recently, photophy-
sics and spectroscopy of the vacuum-isolated jet-cooled
4-cyano-N-dimethylaniline and related molecules have been
reviewed by Phillips.12
22
line and 2-cyanoaniline. Roth et al. in their study of 4-cya-
nophenol have found substantial change in geometry of the
cyano group upon excitation. The strongest effect has been
found in the LIF excitation spectra of ortho-substituted aniline
23
derivatives: 2-cyanoaniline,21 2,6-dicyanoaniline
and 2,6-
dicyano-3,5-dimethylaniline.24 Their LIF excitation spectra
are dominated by long progressions in the C-CN bending
mode forming combinations with other fundamental modes.
Moreover, in the LIF excitation spectrum of 2,6-dicyanoani-
line totally symmetric stretching fundamental mode of cyano
groups have been identified.23
The purpose of this paper is to investigate the potential
energy functions in the S0 and S1 states of 3,5-dicyanoaniline
using ab initio quantum mechanical calculations and IR,
Raman and LIF spectroscopic techniques. Due to molecular
structure of 3,5-dicyanoaniline intramolecular hydrogen bond-
ing in the jet-cooled molecule is impossible. Thus, comparisons
of the spectroscopic properties of 2,6-dicyanoaniline and 3,5-
dicyanoaniline make it possible to reveal these features which
are influenced by intramolecular hydrogen bonding.
The potential energy function for the inversion of the amino
group in aniline and its derivatives substituted in the benzene
ring is symmetrical with respect to reflections in the molecular
plane. The lowest state vibrational wave function for the inver-
sion mode is also symmetrical with respect to this reflection.
For the vacuum-isolated jet-cooled aniline in its lowest vibra-
tional level the molecular geometry is bipyramidal, with C2v
symmetry. From the spectroscopic evidence the tunneling
splitting for aniline is 40 cmꢀ1 and 12 cmꢀ1 for deuterated
Possible effects due to changes in the hybridization on the
carbon atom of cyano group were suggested by Sobolewski
and Domcke.17 Although this latter change of hybridization
is not responsible for the appearance of dual fluorescence of
4-cyano-N-dimethylanilines, as originally suggested by Sobo-
lewski and Domcke, it may, however, contribute to the pre-
sence of progressions in bending mode of CCN group in
y Electronic supplementary information (ESI) available: Analysis of
the LIF spectra of 3,5-DCA(–NH2) and 3,5-DCA(–ND2). See
http://www.rsc.org/suppdata/cp/b3/b305797f/
4096
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107
This journal is # The Owner Societies 2003
DOI: 10.1039/b305797f

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aniline (ꢀND2).9,11 From the quantum mechanical ab initio
studies we find that for aniline derivatives containing substitu-
ent in the benzene ring the barrier for inversion is usually
lower,21 thus, the resultant tunneling splitting is larger and
the tunneling rate should also increase. On the other hand,
substitutions of the hydrogen atoms in the amino group by
heavier molecular groups reduce the tunneling rates and aver-
age hybridization of the nitrogen atom becomes pyramidal.
Similar effects may result from the hydrogen bonding and
perturbing by external fields in crystalline phases.
In this paper we present: (i) the results of experimental stu-
dies of the LIF excitation spectra for 3,5-dicyanoaniline and its
deuterated (ꢀND2) derivative in the supersonic molecular
beams and the corresponding ground state IR and Raman
spectra, (ii) ab initio calculations of the molecular geometry
and harmonic vibrational frequencies for these geometries
which correspond to the stationary points of the potential
energy functions for the ground electronic state and the lowest
electronic excited state, (iii) correlations of the vibrational fre-
quencies in the ground state and S1 excited state of 3,5-DCA,
and (iv) modeling of the intensity distributions in the LIF exci-
tation spectra of 3,5-DCA(–NH2), 3,5-DCA(–ND2) taking
into account the displacements of normal coordinates,
frequency changes and mode mixing.
Quantum mechanical calculations using the GAUSSIAN98
program package27,28 were performed at the Academic Com-
puter Center on a SGI ORIGIN2800 computer. The ab initio
HF and CIS methods with 6-31++G** basis functions were
used for calculations of molecular structure and vibrational
frequencies at equilibrium geometries. Vibrational frequencies
were calculated for 3,5-cyanoaniline and its deuterated
(ꢀND2) forms in the ground and S1 excited states. Calculated
normal mode coordinates were used to derive the rotation
matrices of the normal mode coordinates for each compound.
The rotation matrices of the normal mode coordinates (the
Dushinsky matrices) were calculated for electronic transitions
and for each pair of deuterium exchanged species.
Calculations of the multimode Franck–Condon factors and
relative transition probabilities for the LIF excitation spectra
have been performed using the recurrence equations derived
by Ruhoff.31,32 For this purpose appropriate programs in the
MATLAB calculation environment have been developed.
The vibrational band intensities were calculated in two ways:
(i) using all input data from the ab initio quantum mechanical
calculation, and (ii) the displacement parameters for the totally
symmetric modes have been optimized to get the best agree-
ment with the experiment using all other parameters from
the ab initio calculation. Minimization procedures for the
sum of the least squares were used for evaluation of the displa-
cement parameters of the normal vibrational modes for the
3,5-DCA(–NH2) and 3,5-DCA(–ND2) molecular species.
2. Experimental
For the excitation of the fluorescence emission, a HY 400
Nd:YAG laser and a dye laser DY 300 (Lumonics) were used.
The 3,5-DCA(ꢀNH2) and 3,5-DCA(ꢀND2) fluorescence was
excited by the frequency doubled (BBO) output of the dye laser
operating on DCM and rhodamine 640 supplied by Exciton.
The pulses at 10 Hz were 10 ns in duration and had 20 mJ
(or less) of energy per pulse, with a band width of about 0.1
cmꢀ1. The laser beam was introduced into a vacuum chamber
and crossed the seeded supersonic expansion 10 mm down-
stream from an orifice having 0.5 mm internal diameter. Jet
expansion was controlled by a General Valve device. A
CAMAC 712 pulse analyzer was used to measure pulses deliv-
ered from a Thorn EMI 9558QB photomultiplier cooled at
245 K. Helium carrier gas was dried using a trap with liquid
nitrogen. The sample injection system was heated at 120 ꢁC.
Synthesis of 3,5-cyanoaniline was accomplished by the fol-
lowing procedure. A mixture of 3,5-dibromoaniline (6.3 g,
25.1 mmol), copper(I) cyanide (8.6 g, 95.6 mmol) and
dimethylformamide (15,0 mL) was refluxed with vigorous stir-
ring for 4 h. The mixture was cooled and a solution of sodium
cyanide (17.2 g, 350.9 mmol) in water (60.0 mL) was added. A
precipitated solid was filtered off and washed with water. The
filtrate was extracted with ethyl acetate (4 ꢂ 30.0 mL), dried
over magnesium sulfate and the solvent was removed under
reduced pressure. Combined portions of crude 3,5-dicyjanoa-
niline were purified by vacuum sublimation and then by
crystallization from petroleum ether, yield 0.9 g (25%);
mp 192–194 ꢁC. These values compare favourably with avail-
able data25,26 mp. 189–190 ꢁC.
3. Results and discussion
3.1. Molecular geometry
According to Gordon et al.,9 in the aniline molecule in its
ground state the barrier height for inversion is 526 cmꢀ1 and
the tunneling splitting amounts 41 cmꢀ1. The ab initio mole-
cular geometries for the ground electronic state (HF/
6-31++G**) and the lowest S1 electronic state (CIS/6-
31++G**) for 3,5-DCA, 2,6-DCA and aniline have been com-
pared in Table 1. The HF/6-31++G** calculation predicts
pyramidal geometry of the amino group in the ground state
of 3,5-DCA. This result resembles the situation for the aniline
molecule. Due to high tunneling rate for inversion in vacuum
isolated aniline derivatives an average molecular geometry is
bipyramidal with C2V symmetry. The relevant tunneling rates
are higher than 1 psꢀ1. The electronic S0 ! S1 transition rates
for the molecules considered here are order of 0.1 nsꢀ1. To
take into account this effect, resulting from the nuclear
motions, we have also performed the HF calculations of the
nuclear geometries imposing planarity on the aniline, 3,5-
DCA and 2,6-DCA molecules in their ground electronic state.
For these reasons in Table 1 the results of the HF/6-31++G**
calculated bond lengths and angles for the stationary saddle
point corresponding to the planar geometry of 3,5-DCA have
been included. The results concerning the ground state vibra-
tions of the 3,5-DCA molecule calculated for these two sta-
tionary points, corresponding to the pyramidal and planar
(bipyramidal) geometries, have been compared in Tables 2
and 3 for the native and deuterated forms.
From Tables 2 and 3 we find that the frequencies of vibra-
tional modes calculated for the pyramidal and planar 3,5-
DCA molecules are very similar when the same isotopic species
are compared. Large discrepancies occur for the inversion
mode. This is because the inversion motion in the double mini-
mum potential is highly anharmonic. The calculated frequen-
cies for the inversion mode merely reflect the details of the
potential function for inversion at the potential well and at
the barrier top (imaginary frequencies). Using the HF/6-
31++G** method the barrier height for the inversion motion
for 3,5-DCA was evaluated as 230 cmꢀ1. The barrier for the
Spectroscopic data: 1H NMR (500 MHz, DMSO-d6) 6.12 (s,
2H), 7.15 (d, 2H, J ¼ 1.4 Hz), 7.39 (t, 1H, J ¼ 1.4 Hz); 13C
NMR (125 MHz, DMSO-d6) 113.08, 117.72, 120.08, 121.15,
150.13; IR max/cmꢀ1: 435, 672 865, 1328, 1343, 1443 1599,
1630, 2230, 3344, 3463, detailed discussion in the Results and
discussion section.
Deuterium exchange in the 3,5-DCA was achieved by boil-
ing with deuterated ethanol (C2H5OD) with a small amount
of D2O. FTIR spectra of the 3,5-DCA powdered samples,
pressed in KBr pellets, were measured with a Brucker IFS 48
instrument. Raman spectra of the solid samples were measured
using a Digilab 2000 instrument with 1064 nm excitation and
FT detection.
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107
4097

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Table 1 Ab initio geometries for 2,6-dicyanoaniline and 3,5-dicyanoaniline in the S0 (HF/6-31++G**) and S1 (CIS/6-31++G**) states. The
results for planar (pl) and pyramidal (py) molecular geometries of 3,5-dicyanoaniline are given
23
3,5-DCA
S0(py)
2,6-DCA
S0(pl)
Aniline
S0(py)
S0(pl)
1.396
S1(pl)
1.423
S1(pl)
1.438
S1(pl)
1.435
˚
C1–C2/A
1.393
1.387
1.388
1.075
1.073
1.385
0.996
0.250
1.448
1.136
118.5
120.4
121.5
117.8
37.4
1.409
1.389
1.383
1.075
1.073
1.350
0.993
0
1.394
1.385
1.387
1.077
1.075
1.395
0.996
0.278
—
˚
C2–C3/A
1.378
1.388
1.074
1.072
1.368
0.991
0
1.408
1.409
1.073
1.074
1.332
0.994
0
1.409
1.402
1.072
1.074
1.316
0.997
0
1.364
1.403
1.071
1.074
1.312
1.004
0
˚
C3–C4/A
˚
C2(3)–H/A
˚
C4–H/A
˚
C1–NH2/A
˚
N–H/A
l
C–CN/A
˚
1.448
1.136
1.4286
1.1412
1.439
1.138
116.8
121.1
121.2
118.5
0
1.418
1.144
—
—
˚
CꢃN/A
—
C6–C1–C2/ꢁ
C1–C2–C3/ꢁ
C2–C3–C4/ꢁ
C3–C4–C5/ꢁ
y/ꢁ
118.3
122.1
120.3
118.8
120.4
120.9
118.7
41.3
119.3
119.6
120.4
120.8
0.0
120.3
121.7
117.6
0
118.3
120.0
121.3
0
119.1
119.4
122.6
0
H–N–H/ꢁ
C–N–H/ꢁ
C–C–N–H/ꢁ
C2(1)–C3(2)–CN/ꢁ
C–CꢃN/ꢁ
112.5
116.2
23.5
117.8
121.1
0
117.7
121.1
0
118.0
121.0
0
118.4
120.8
0
111.6
115.0
25.5
116.5
121.7
0
119.0
179.7
118.9
179.8
120.5
179.2
118.9
178.1
117.7
177.0
—
—
—
—
inversion on the amino group in 3,5-DCA assumes intermedi-
ate value between those for aniline and 2,6-dicyanoaniline. The
corresponding barriers calculated at the HF/6-31++G** level
for aniline and 2,6-DCA were found equal 408 cmꢀ1 and
0 cmꢀ1, respectively.
calculated vibrational frequencies in Table 2 are numbered
according to increasing frequencies for each molecular species,
each electronic state and each stationary point (except for the
case of imaginary frequencies of the inversion motion at planar
molecular geometry).
Geometry optimization for 3,5-DCA in the excited S1 state
has been performed without any constraints concerning mole-
cular symmetry. The results presented in Table 1 show that in
the S1 excited state of 3,5-DCA the amino group assumes pla-
nar geometry, i.e. the potential energy function for inversion
has single minimum. Moreover, upon electronic excitation
remarkable changes of the interatomic distances occur espe-
cially in the vicinity of the amino group. For the changes of
interatomic distances assuming the nonplanar geometry of
the initial ‘equilibrium’ ground state we find: C1–C2 + 3.0
pm, C2–C3 and C3–C4 + 2.1 pm, C1–NH2 ꢀ 5.3 pm, C–
CN ꢀ 1.9 pm. Corresponding changes of the molecular geome-
try calculated taking as a reference the planar geometry of the
molecule in the initial ground state are the following: C1–
C2 + 2.7 pm, C2–C3 + 3.0 pm, C3–C4 + 2.1 pm, C1–NH2 ꢀ 3.6
pm, C–CN ꢀ 1.9 pm.
Table 2 gives also measured IR and Raman band frequen-
cies and the calculated harmonic frequencies for the 3,5-
DCA(ꢀNH2) whereas Table 3 contains experimental IR and
Raman band frequencies for the 3,5-DCA (ꢀND2) form. Cor-
relations between the vibrational frequencies of the 3,5-DCA
(ꢀNH2) and 3,5-DCA (ꢀND2) species in their equilibrium
and planar geometries have been performed using the rotation
matrices of the normal coordinates for corresponding forms of
3,5-DCA in the S0 electronic state. For the ab initio HF fre-
quencies calculated using the 6-31++G** basis set the scaling
factors were found to be slightly different for the in-plane
vibrational modes ( f ¼ 0.915 ꢀ 7.5 ꢂ 10ꢀ6ncalc) and the out-
of-plane vibrations.21,29 For the latter, a constant factor of
0.89 has been used. The same scaling factors have been applied
to the vibrational frequencies in the S1 electronic state,
calculated using the CIS/6-31++G** method. For planar
geometry of the 3,5-DCA molecule a single imaginary fre-
quency corresponding to the inversion mode has been calcu-
lated (i 380.6 cmꢀ1 for ꢀNH2 form and i 293.0 cmꢀ1 for
ꢀND2 form).
3.2. Vibrational modes
There are 42 normal modes of vibration in the 3,5-DCA mole-
cule, 29 in-plane (15 a1 , 14 b2 in C2v) and 13 out-of-plane
(8 b1 , 5 a2 in C2v) vibrational species. The out-of-plane modes
belong to the b1 and a2 species. Raman spectra of the crystal-
line samples of 3,5-DCA(ꢀNH2) and 3,5-DCA(ꢀND2) at
room temperature are shown in Fig. 1. In Table 2 the ground
state vibration frequencies for 3,5-DCA are compared with
the calculated ab initio HF/6-31++G** frequencies. The
Since IR and Raman experimental spectroscopic data refer
to crystalline samples whereas calculations of the vibrational
frequencies have been performed for the isolated molecule,
the quality of correlation is obviously affected by contributions
from the hydrogen bonding and matrix effects as well. Thus,
we were unable to assign transition involving the inversion
modes in our IR spectra. The frequencies for stretching
4098
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107

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Table 2 Assignment and correlations of the molecular vibrations in the S0 state for 3,5-DCA(ꢀNH2). The calculated ab initio HF/6-31++G**
vibrational frequencies (in cmꢀ1) were scaled as described in the text. The calculated vibrational frequencies are numbered according to increasing
frequencies. IIR and ARam represent calculated IR and Raman intensities. Normal modes are identified by their reference to the Varsanyi descrip-
tion for substituted benzene derivatives29
HF/6-31++G**
3,5-DCA(–NH2)
Experimental
IR
3,5-DCA(–NH2): planar
Raman
a
a
ꢀ
ꢀ
ꢀ
ꢀ
n
No
nscaled
IIR
10.0
IRam
No
nscaled
IIR
10.0
IRam
Assign
n
I/InCN
<0.1
I/InCN
a1
(a0)
2
9
118.2
445.0
533.0
550.3
967.3
988.1
1160.3
1315.4
1442.8
1606.5
1638.4
2327.6
2997.6
3024.1
3380.0
110.5
215.1
436.1
507.0
614.8
666.5
879.4
896.3
160.8
243.1
377.5
624.4
902.0
170.2
361.8
447.2
628.0
904.5
1062.1
1142.7
1175.2
1303.2
1470.5
1617.5
2327.0
2997.4
3475.1
229.1
82.0
93.0
235.5
474.5
245.6
136.3
745.4
2.9
2
9
118.4
445.4
530.4
229.1
82.0
93.0
235.5
474.5
244.4
136.3
744.3
2.9
9a C–(CN) bend.
6a
11.9
16.7
126.7
2.0
11.9
16.7
126.7
2.0
459
458
547
0.15
0.40
13
14
23
24
27
30
31
33
35
37
39
40
41
1
13
14
23
24
27
30
31
33
35
37
39
40
41
1
CN bend
I(–NH2), ring def.
540.1
965.6
991.2
1
12
975
1010
1158
1343
1443
1599
1630
2230
3078
3090
3344
<0.1
0.1
0.1
0.6
0.6
1.4
2.7
1.0
0.3
0.3
1.4
976
1008
0.34
0.06
17.7
23.8
58.6
42.9
96.2
186.6
15.5
3.8
17.7
23.8
58.6
42.9
96.2
186.6
15.5
3.8
1161.2
1325.4
1444.9
1602.8
1626.7
2327.2
2997.1
3025.2
3427.1
110.7
18a
13
1343
1445
1599
1625
2232
3074
3090
0.23
0.03
0.07
0.04
1.00
0.13
0.07
19a
8a
260.8
147.9
3035.0
387.6
200.7
483.2
0.0
260.8
147.9
3035.0
387.6
200.7
483.2
0.0
NH2 scissoring
nCN
nCH
nCH
nNH2
11
1.0
1.0
49.7
5.4
49.7
4.6
b1
(a0)
5
6.1
21.1
61.7
42.7
61.7
5.8
5
213.3
437.8
3.3
18.3
24.3
1.9
10b
16b
227
436
523
616
0.02
0.03
0.06
0.05
10
12
16
18
19
21
3
21.8
165.0
30.0
23.9
61.8
6.5
10
12
16
18
19
21
3
14.5
277.2
0.0
435
521
604
672
865
839
0.4
<0.1
0.1
i 369.0
609.7
667.0
ring def., I(–NH2)
ring, CN bend.
50.8
5.8
24.8
55.1
10.9
0.0
4
0.6
0.9
0.0
0.0
872.4
887.1
0.0
1.3
5
17b
0.1
a2
(a00)
0.6
3.2
7.3
161.4
306.5
0.0
2.1
10a
T(–NH2)
6
17.1
0.2
6
0.0
0.0
8
100.4
104.6
1.3
9
378.3
625.2
102.2
100.1
1.3
16a
CN bend
374
926
0.12
0.03
17
22
4
0.1
0.0
17
22
4
0.0
0.0
893.5
170.5
17a
9b C–(CN) bend
b2
(a00)
0.8
4.7
111.7
0.0
0.4
7.1
123.2
0.0
7
7
361.1
446.6
C–(NH2) bend
6b
11
15
20
25
26
28
29
32
34
36
38
42
0.0
0.7
33.1
23.8
30.3
36.1
4.5
11
16
20
25
26
27
29
32
34
36
38
42
0.0
0.5
29.9
24.1
31.9
22.7
5.7
627.5
902.5
CN bend
7b
4.8
2.7
855
1103
0.2
<0.1
26.3
4.8
1032.5
1141.2
1158.7
1302.9
1466.0
1612.4
2326.6
2996.8
3541.1
25.8
0.0
NH2 bend, 14
18b
14, NH2 bend
41.3
2.1
3.3
2.1
69.1
3.2
7.8
2.1
1176
1328
1464
0.1
0.7
1178
1333
1464
1587
2242
3040
0.08
0.05
0.01
0.07
0.34
0.07
3
19b
6.2
5.7
2.3
6.7
<0.1
15.7
60.8
0.7
387.7
973.6
207.3
138.7
22.2
61.2
0.9
341.2
955.8
214.9
119.4
8b
nCN
nCH, 20b
nNH2
2242
3045
3463
0.2
0.3
1.2
33.1
55.9
a
IRam ¼ K(n)ARam (from calculations); K(n) ¼ 2.397497 ꢂ 10ꢀ15 (n0 ꢀ nscaled)4/(1 ꢀ exp(ꢀnscaled/207.23)); n0 ¼ 9398.5 cmꢀ1 (incident radiation
energy).
N-H modes derived from the IR spectra have slightly lower
frequencies than the calculated ones. These last differences
may result from hydrogen bonding in crystalline sample.
Moreover, those nearest to the first excited electronic state
have low transition probabilities. The results of CIS/6-
31++G** calculation lead to similar conclusion, the excited
states with large oscillator strength should occur at energies
higher than 45 000 cmꢀ1
.
3.3. LIF excitation spectra
The LIF excitation spectra of 3,5-DCA(ꢀNH2) and its deut-
erated (ꢀND2) form are given in Figs. 2a, and 2b, respectively,
together with probable assignments of the fundamental transi-
tions. The electronic origin for the S0(A1) ! S1(B2) electronic
transition in the 3,5-DCA molecule is at 30295.1 cmꢀ1. Due
to deuterium exchange the electronic origin shifts to lower
frequencies by ꢀ9.4 and ꢀ16 cmꢀ1 for ꢀNHD and ꢀND2
forms, respectively. These shifts are similar to those found for
2,6-dicyanoaniline23 and 2,6-dicyano-3,5-dimethylaniline,24
ꢀ9 and ꢀ18 cmꢀ1 for ꢀNDH and ꢀND2 forms, respectively
Vapor phase absorption spectrum of 3,5-DCA at 120 ꢁC shows
absorption bands of comparable intensities centered at 31 000
and 41 000 cmꢀ1 and strong absorption band centered at
45 500 cmꢀ1. This observation is qualitatively reproduced by
the TD DFT B3LYP/6-31++G** calculation. In this evalua-
tion three lowest excited states are as follows: S1 , 31 551 cmꢀ1
( f ¼ 0.062), S2 , 39 455 cmꢀ1 (f ¼ 0.033) and S3 , 41 616 cmꢀ1
( f ¼ 0.003). These results suggest rather large separation of
the S1 (B2) excited electronic state and the higher excited states.
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107
4099

View Article Online
Table 3 Assignment and correlations of the molecular vibrations in the S0 state for 3,5-DCA(ꢀND2)
HF/6-31++G**
Experimental
3,5-DCA(–ND2)
3,5-DCA(-ND2) planar
IR
nꢀ
Raman
I/InCN
IR
nꢀ
Raman
I/InCN
Noa
nꢀscaled
IIR
10.0
IRam
No
nꢀscaled
IIR
IRam
Assign
b
b
a1
(a0)
2 (2)
11 (9)
118.0
445.7
532.8
506.2
954.1
981.2
1135.2
1338.4
1443.0
1609.6
1210.2
2327.6
2997.6
3024.1
2466.3
109.2
206.3
447.5
407.9
612.1
666.5
878.9
896.2
154.4
181.0
376.9
623.9
901.8
166.0
335.9
444.9
618.6
924.3
847.1
1145.2
1134.7
1301.9
1452.9
1614.4
2327.0
2997.4
2591.8
229.4
116.8
142.7
110.0
177.3
540.3
15.7
562.5
5.8
2
118.2
445.4
533.0
10.1
225.0
120.3
160.4
110.2
313.9
445.7
13.5
600.4
11.5
410.3
356.4
3048.9
398.6
200.2
481.4
0.0
9a C–(CN) bend
6a
1.4
0.6
11
14
13
24
25
26
31
32
34
29
37
41
42
38
1
0.6
0.9
2.3
2.4
24.2
7.8
152.1
93.3
268.1
28.5
15.2
4.9
1.1
99.4
5.5
1.1
10.9
154.9
0.5
23.7
58.4
10.7
0
458
0.2
<0.1
<0.1
0.1
461
0.17
0.26
0.12
0.30
0.20
0.02
0.13
0.05
0.18
0.17
? 0.88
0.34
0.10
0.20
14 (13)
13 (12)
24 (23)
25 (24)
27 (27)
31 (30)
32 (31)
34 (33)
29 (35)
37 (37)
41 (39)
42 (40)
38 (41)
1 (1)
CN bend
ring def., I(–ND2)
538
522
539
522
3.1
9.6
520.4
954.7
980.9
1
12
967
968
14.1
8.1
1006
1152
1370
1447
1604
1203
2231
3074
3086
2446
0.1
1001
1148
1370
1449
1604
1214
2232
3070
3087
2448
1132.4
1353.6
1446.8
1606.3
1205.6
2327.2
2997.0
3025.2
2498.7
109.6
18a
13
<0.1
0.6
119.6
61.5
198.1
21.0
15.5
3.8
19a
8a
1.3
2.9
357.5
334.0
3034.4
389.6
202.3
458.4
0.0
ND2 bend.,
nCN
nCH
nCH
nNH2
11
0.2
1.0
0.3
0.3
0.9
47.5
5.3
0.6
b1
(a0)
6 (5)
12 (10)
9 (14)
9.9
29.9
33.1
58.3
52.8
5.8
5
207.8
436.7
23.9
22.6
0.0
10b
16b
211
435
0.04
0.04
25.5
169.5
8.1
12
7
434
0.5
i 284.3
607.3
666.9
I(–ND2), ring def.
ring, CN bend.
16 (16)
18 (18)
20 (19)
21 (21)
3 (3)
16
18
20
21
3
52.4
5.8
600
668
853
839
0.3
0.7
0.6
0.7
615
839
0.08
22.8
61.1
6.9
4
0.0
0.0
872.2
887.1
0.0
1.3
5
17b
<0.01
a2
(a00)
2.3
6.7
10.0
11.9
96.7
106.1
0.0
159.0
221.4
0.0
2.6
10a
T(–ND2)
5 (6)
8 (8)
6
0
0.1
0.0
9
377.0
624.3
0
96.7
103.1
1.3
16a
CN bend.
374
945
0.14
0.04
17 (17)
22 (22)
4 (4)
17
22
4
0
0.1
1.1
893.0
165.6
0
17a
9b C–(CN) bend.
b2
(a00)
91.7
2.0
0.4
6
119.4
0.0
7 (7)
3.1
0.1
8
332.2
444.0
C–(ND2) bend.
6b
10 (11)
15 (15)
23 (20)
19 (25)
28 (26)
26 (28)
30 (29)
33 (32)
35 (34)
36 (36)
40 (38)
39 (42)
33.1
19.5
57.5
1.3
10
15
23
19
28
27
30
33
35
36
40
39
0
31.8
21.2
51.9
1.3
452
880
0.04
1.0
8.3
616.2
920.7
826.3
0.8
7.8
0.2
51.3
38.1
3.4
2
CN bend
7b
880
1147
1326
0.2
0.1
0.7
<0.01
0.1
ND2 bend., 14
18b
14, ND2 bend.
36.1
32.5
2.5
0.0
1147.2
1132.5
1301.9
1452.2
1610.2
2326.6
2996.8
2647.7
1.1
20.2
2.1
19.3
2.1
1177
1329
0.02
0.03
3
19b
4.5
2.9
2.9
16.9
60.7
0.7
390.1
973.6
208.1
115.3
22.2
61.2
0.9
28.8
345.2
955.2
215.3
102.1
8b
1583
2243
3025
2597
0.08
? 1.00
0.05
nCN
nCH, 20b
nNH2
2243
2589
0.7
2.3
17.5
0.06
a
In parentheses the numbering of the corresponding normal modes for 3,5-DCA(-NH2) is given (column 2 in Table 2). b IRam ¼ K(n)ARam (from
calculations); K(n) ¼ 2.397497 ꢂ 10ꢀ15 (n0 ꢀ nscaled)4/(1 ꢀ exp(ꢀnscaled/207.23)); nꢀ0 ¼ 9398.5 cmꢀ1 (incident radiation energy).
and about two times larger than the deuteration shifts for
3-cyanoaniline ꢀ5 and ꢀ7 cmꢀ1, respectively.21
Table Ib (ESI)y lists vibrational band positions in the LIF
excitation spectra of 3,5-DCA(ꢀND2) and 3,5-DCA(ꢀNHD),
their relative integral intensities (column 6) and the assignment
of the observed transitions based on the results of the CIS/6-
31++G** calculation. Table Ib (ESI) is organized in the
same way as Table Ia (ESI). The relative band intensities
for 3,5-DCA(ꢀNHD) are listed relative to the 00 transition
for the 3,5-DCA(ꢀND2) species. The last two columns give
the calculated transition intensities in the LIF excitation spec-
trum of 3,5-DCA(ꢀND2).
The vibrational lines in the LIF excitation spectra of 3,5-DCA
have FWHH around 1.4–1.7 cmꢀ1. In the spectral range 2250
cmꢀ1 above the electronic origin for 3,5-DCA almost 140 spectral
lines have been found. Table Ia of the electronic supplemen-
tary information (ESI)y lists vibrational band positions in
the LIF excitation spectrum of 3,5-DCA(ꢀNH2), their relative
integral intensities (column 6) and the assignment of the transi-
tions based on the results of the CIS/6-31++G** calculation
(columns 4 and 5). In column 2 the analysis of the LIF excitation
spectrum in terms of fundamental experimental frequencies is
given. In column 3 the differences between the observed fre-
quencies and the assigned combinations of fundamental fre-
quencies are shown. For correlation of vibrational modes in
the ground S0 and excited S1 states, the Dushinsky matrices,
Rij , describing transformations of normal modes upon electro-
nic excitation were employed. (See Appendix A and Table 4.)
Correlations between the normal modes calculated for the
ground S0 electronic state and the first excited electronic state
S1 of the planar 3,5-DCA molecule (for the assumed C2v sym-
metry in both states) are displayed in Table 4. In Table 4 the
four diagonal blocks of the Dushinsky matrix are shown:
a1 ꢂ a1 , b1 ꢂ b1 , a2 ꢂ a2 and b2 ꢂ b2 . The off-diagonal blocks
are not shown, because they are zero blocks. The vibrational
modes are numbered according to increasing frequency in
4100
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107

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Fig. 1 Raman spectra of 3,5-DCA(ꢀNH2) and 3,5-DCA(ꢀND2) in crystalline phases at room temperature.
each electronic state. The correlations shown in Table 4 are
based on the largest projections of normal coordinates in the
corresponding electronic states. The vibrational modes are
arranged in such a way that the largest overlap appears on
the diagonal of the matrix. In Table 4 the displacements of
the normal coordinates under S0 ! S1 excitation for the totally
symmetric vibrations a1 are included.
Table 4 gives explicit evidence of mode mixing upon electro-
nic excitation. A mode mixing is predicted among skeleton
modes, especially among modes revealing large frequency
Fig. 2 LIF excitation spectrum of jet-cooled 3,5-dicyanoaniline in native form 3,5-DCA(ꢀNH2) (a) and in deuterated form 3,5-DCA(ꢀND2)
(b). The bands are indicated by their frequencies in cmꢀ1 relatively to the band origin: 30295.1 cmꢀ1 for 3,5-DCA(ꢀNH2) and 30279.2 cmꢀ1
for 3,5-DCA(ꢀND2).
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107
4101

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Table 4 Normal mode displacements and diagonal blocks a1 ꢂ a1 , b1 ꢂ b1 , a2 ꢂ a2 and b1 ꢂ b1 of the Dushinsky matrix for the S0(A1) ! S1(B2)
electronic transition calculated using HF/6-31++G** and CIS/6-31++G** methods for the 3,5-DCA molecule. The vibrational modes and
stationary point molecular geometries are calculated for planar 3,5-DCA molecules in the S0 and S1 electronic states
a1
S0
pl
No
2
2
10
9
13
13
14
14
23
23
24
24
28
27
30
30
31
31
33
33
35
35
37
37
39
39
40
40
41
41
Ki
S1
ꢀ0.249
0.254
3
12
17
14
23
24
27
30
31
34
35
37
39
40
41
ꢀ100
ꢀ2
1
ꢀ1
ꢀ1
1
ꢀ1
ꢀ1
2
2
ꢀ100
ꢀ3
ꢀ7
7
1
ꢀ1
2
2
ꢀ1
ꢀ2
4
ꢀ0.066
0.220
0.137
3
7
1
ꢀ100
ꢀ6
ꢀ2
1
1
3
1
ꢀ2
ꢀ1
ꢀ1
ꢀ99
ꢀ3
ꢀ2
1
1
1
ꢀ99
ꢀ4
ꢀ7
1
2
ꢀ8
19
97
ꢀ6
3
5
ꢀ3
4
ꢀ2
3
1
0.052
ꢀ98
19
1
ꢀ3
7
ꢀ6
ꢀ0.043
ꢀ0.124
ꢀ0.040
ꢀ0.015
0.008
a1
2
1
ꢀ4
ꢀ8
ꢀ13
98
ꢀ4
1
1
ꢀ1
ꢀ2
1
2
1
2
ꢀ1
1
74
ꢀ67
ꢀ72
ꢀ15
7
ꢀ1
ꢀ1
ꢀ6
ꢀ4
2
6
ꢀ66
ꢀ3
10
ꢀ14
12
1
1
7
3
ꢀ2
ꢀ1
ꢀ2
ꢀ1
ꢀ4
ꢀ3
1
14
1
ꢀ98
1
0.023
1
1
2
ꢀ100
ꢀ0.007
0.000
0.007
2
ꢀ1
1
ꢀ1
ꢀ99
ꢀ16
16
ꢀ99
ꢀ1
ꢀ100
b1
S0
pl
No
1
1
5
5
12
10
8
12
15
16
18
18
19
19
21
21
S1
2
5
ꢀ98
16
ꢀ15
6
9
2
4
2
4
3
1
4
1
ꢀ98
3
ꢀ3
ꢀ59
14
ꢀ2
13
7
ꢀ1
52
ꢀ3
ꢀ45
71
ꢀ20
ꢀ97
ꢀ2
9
ꢀ54
17
ꢀ20
3
b1
ꢀ4
ꢀ1
10
ꢀ4
2
1
ꢀ3
15
8
ꢀ2
5
ꢀ77
ꢀ4
18
44
68
ꢀ7
7
2
20
22
1
ꢀ2
2
2
13
ꢀ1
ꢀ97
4
ꢀ4
ꢀ1
ꢀ1
1
ꢀ100
a2
S0
pl
No
3
3
6
6
9
8
17
17
22
22
S1
1
10
6
ꢀ85
7
ꢀ7
50
27
13
ꢀ1
47
83
19
21
48
ꢀ14
ꢀ35
92
a2
ꢀ52
ꢀ6
ꢀ75
ꢀ32
5
7
10
16
18
ꢀ87
b2
S0
pl
No
4
4
7
7
11
11
16
15
20
20
25
25
26
26
27
28
29
29
32
32
34
34
36
36
38
38
42
42
S1
4
9
ꢀ100
ꢀ3
3
3
ꢀ1
1
3
ꢀ1
1
ꢀ100
ꢀ7
ꢀ99
ꢀ1
ꢀ8
3
ꢀ4
2
ꢀ2
0
1
0
1
ꢀ3
3
ꢀ1
11
19
21
25
26
33
29
32
28
36
38
42
ꢀ3
7
4
8
2
ꢀ2
ꢀ4
ꢀ4
ꢀ5
ꢀ13
ꢀ27
81
ꢀ4
4
ꢀ99
ꢀ2
ꢀ9
7
3
0
ꢀ2
1
ꢀ6
ꢀ3
38
1
1
1
ꢀ99
ꢀ4
6
ꢀ5
27
ꢀ1
ꢀ1
1
ꢀ2
1
ꢀ83
ꢀ34
ꢀ24
4
4
ꢀ24
3
3
ꢀ3
ꢀ2
1
2
ꢀ87
ꢀ8
ꢀ18
17
1
ꢀ20
ꢀ38
24
ꢀ1
ꢀ1
1
b2
1
4
ꢀ3
ꢀ4
ꢀ2
2
ꢀ3
ꢀ1
3
26
ꢀ87
ꢀ8
42
ꢀ24
ꢀ25
ꢀ76
ꢀ49
30
ꢀ1
ꢀ1
1
3
3
1
10
34
ꢀ38
7
ꢀ49
60
ꢀ3
4
2
ꢀ5
ꢀ2
1
ꢀ30
1
ꢀ2
ꢀ1
7
ꢀ100
1
ꢀ1
ꢀ2
ꢀ2
ꢀ100
ꢀ100
change upon electronic excitation, and the modes which
engage motions of the amino group. Strong mixing is predicted
to occur between some out-of-plane modes (see b1 ꢂ b1 and
a2 ꢂ a2 submatrices) as well as between in-plane modes of b2
symmetry.
(ꢀNDH and ꢀND2) forms are compared with the vibrational
frequencies calculated using the CIS/6-31++G** method for
these three isotopic species.
The LIF excitation spectrum of 3,5-DCA allows for
assignment of 9 normal vibrations of the a1 symmetry. The
1
In Table 5 the fundamental frequencies identified in the LIF
excitation spectra of 3,5-DCA in native and deuterated
remarkable feature of the1A0 ! B2 LIF excitation spectra of
3,5-DCA is vibronic activity of the vibrational modes which
4102
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107

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Table 5 Assignments and correlations of the fundamental vibrational frequencies observed in the LIF excitation spectra of 3,5-DCA(–NH2), 3,5-
DCA(–NDH) and 3,5-DCA(–ND2) based on the results of ab initio CIS/6-31++G** calculations for the vibrational frequencies and displacements
of normal coordinates. The calculated vibrational frequencies are numbered according to increasing frequencies in each electronic state and isotopic
species
CIS/6-31++G**
No
H2
nꢀscaled/cmꢀ1
ꢀNH2
nꢀLIF/cmꢀ1
ꢀNH2
100I/I00
S0
Nr
C2v
HD
D2
ꢀNHD
ꢀND2
ꢀNHD
ꢀND2
H2
HD
D2
2
9
a1
3
12
17
14
23
24
27
30
31
34
35
37
39
40
41
2
3
12
17
14
24
25
27
30
31
33
34
37
40
41
38
2
3
12
17
14
24
25
27
31
32
35
29
37
41
42
38
2
115.2
444.5
531.4
521.1
939.4
971.5
1170.7
1335.7
1404.1
1545.3
1643.4
2289.2
3017.4
3018.9
3393.1
94.4
115.1
444.4
530.8
513.4
940.2
971.3
1157.4
1332.0
1383.1
1494.2
1528.o
2289.2
3017.4
3018.9
2545.5
93.6
115.0
444.4
530.6
505.8
934.7
963.7
1120.8
1343.7
1435.9
1556.0
1214.6
2289.2
3017.4
3018.9
2476.1
92.8
110.7
446.1
517.6
508.5
956.5
946.9
1173.2
1312.0
1390.3
110.5
445.9
509.2
506.4
958.0
110.5
446.0
506.3
503.8
960.3
944.3
19
62
19
32
43
13
9
20
65
21
37
51
17
63
15
35
49
11
13
14
23
24
27
30
31
33
35
37
39
40
41
1
3
25
1423.7
29
b1
5
5
5
5
177.1
479.1
293.7
510.9
340.3
669.7
868.5
81.3
174.6
474.0
244.9
510.4
339.9
669.7
868.5
80.9
172.0
470.0
231.4
510.5
340.1
669.7
868.5
80.4
10
12
16
18
19
21
3
13
7
13
7
13
7
I20 904.1
527.0
757.0
672.5
527.5
28
2
29
46
9
15
8
15
8
16
10
20
22
1
20
22
1
20
22
1
a2
6
10
6
10
6
8
370.9
199.4
516.9
596.0
167.7
368.8
396.1
618.2
879.2
977.2
1096.5
1525.7
1289.7
1444.1
1225.8
2263.2
3014.0
3508.8
347.4
198.6
514.2
580.2
165.4
351.0
394.8
612.5
851.9
906.6
1085.1
1566.1
1276.8
1443.5
1184.8
2263.1
3014.0
3455.5
286.2
197.8
508.9
566.3
163.2
336.4
394.1
606.7
897.6
817.0
1083.8
1522.1
1278.0
1443.3
1172.6
2263.1
3014.0
2623.7
8
6
226.1
520.4
224.2
513.8
222.5
509.2
6
25
5
23
17
22
4
16
18
4
16
18
4
15
18
4
b2
7
9
9
9
372.6
375.6
357.4
373.5
1
6
11
15
20
25
26
28
29
32
34
36
38
42
11
19
21
25
26
33
29
32
28
36
38
42
11
19
21
23
26
35
29
32
28
36
39
42
11
19
23
21
26
34
30
33
28
36
40
39
374.6
831.7
7
941.6
719.4
28
34
involve the aromatic ring. Similar ring mode activities have
been found for 3-cyanoaniline.21
distribution in the S0 ! S1 excitation spectra. Evaluation of
S2–S1 vertical energy gaps has been done using CIS/6-
31++G** calculation for S1 excited state geometry and TD
B3LYP/6-31++G** calculation for S0 geometry. These eva-
luations predict 0.93 eV using TD B3LYP method and 0.63
eV from CIS calculation.
Characteristic features of the LIF excitation spectra of ani-
line and its derivatives are vibronic transitions involving the
inversion mode due to the amino group. The fundamental
transition I10 involving one quantum excitation in the inversion
mode in the upper electronic state is forbidden. Significant
intensity is observed for the overtone transition I20. One should
note here that the forbideness of the I10 transition and the
allowance of the I20 transition indicates that: (1) these mole-
cules have bipyramidal structure and (2) the potential energy
functions in the ground and excited states being sym-
metrical relative to the molecular plane are very different and
anharmonic.
There are eight other frequencies 226 (a2), 373 (b2), 376 (b2),
520 (a2), 904 (b1)2, 942 (b2), 1123 cmꢀ1, and probably 195, 527
(b1) cmꢀ1 which should be assigned to modes of other symme-
try species or their overtones. The electronic excited states of
the 3,5-DCA molecule of the C2V symmetry belong
to B2(pp*), A1(pp*), B1(pp*) and A1(sp*/ps*) irreducible
representations. The vibronic coupling of the lowest excited B2
state can be effective due to b2 vibrational modes–coupling
with A2 states, a2 vibrational modes–coupling with B1 states,
and b1 vibrational modes–coupling with A2 states. Moreover,
the a1 vibrational modes can give vibronic coupling of the B2
state with higher (pp*) excited states of the same B2 symmetry.
However, since UV absorption spectra show that the higher
excited states characterized by a large oscillator strength occur
at energies higher than 1 eV above the S1 state, the vibronic
coupling should be of secondary importance for the intensity
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107
4103

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The multidimensional vector Dq representing geometry
Two vibrational transitions in the LIF excitation spectra:
904 and 942 cmꢀ1 significantly change their frequencies upon
exchange of the hydrogen atoms in the amino group on the
deuterium atoms, 672 and 719 cmꢀ1 respectively. The first
band is a possible candidate for the I20 transition involving
inversion motion (b1 symmetry species) and the second may
correspond to the in-plane NH2 asymmetric bending mode
(b2). The value 904 cmꢀ1 is higher than the I02 transitions in
the S1 state of mono-cyanoanilines (807 cmꢀ1 for 4-cyanoani-
line13,18 and 833, 855 cmꢀ1 for 2- and 3-cyanoaniline, respec-
tively.21 The above seems to indicate that the presence of
two cyano groups in the molecule causes further increase of
the amino group I20 transition frequency in the S1 state in
comparison to aniline.
The deuterated form (ND2) spectrum reveals a line splitting
due to Fermi resonance between modes 6a 446 cmꢀ1 and 16a2
2 ꢂ 222.5 cmꢀ1. Similar line splitting is not present in other iso-
topomers because of larger energy difference between 446 and
2 ꢂ 226 cmꢀ1. The mode 16a belongs to the a2 representation
and a significant mixing between this mode and the 10a mode
upon electronic excitation is observed. The corresponding
off-diagonal coefficients equal 0.50 and ꢀ0.52, respectively
(see Table 5).
change upon electronic excitation derived from the ab initio
quantum mechanical calculation is shown in Fig. 5a. To visua-
lize the geometry change in Fig. 5a the coordinates represent-
ing Cartesian displacements of the atoms have been multiplied
by a constant factor of 40. The result is shown in Fig. 5a.
Comparing the data in Table I of the ESI,y e.g. the column
6 (containing the observed relative intensities) and the column
7 (showing the calculated relative intensities based on the
ab initio results) we find that there is a remarkable degree of
correlation between the observed and calculated band intensi-
ties. It should be noticed that in this modeling of the intensity
distributions we have neglected a possible role of the totally
symmetric modes as the modes engaged in the vibronic
coupling between the lowest excited state and higher excited
electronic states of the B2 symmetry.
The predictions of the relative intensity can be even
improved using optimization procedures. In this approach
the intensity distribution in the LIF excitation spectrum was
calculated using the ab initio results except of the displacement
parameters which were fitted to get the intensities as close as
possible to the observed ones. In the last column 8 in Table I
(ESIy) the results of this particular calculation is shown, as
well as in Fig. 4, where intensities from LIF spectrum, ab initio
parameters and fit are compared.
Knowing the displacement parameters for the totally sym-
metric vibrational modes from the fitting and the normal mode
coordinates from the ab initio quantum mechanical calcula-
tions, the vectors Dq have been calculated using eqn. (A6)
given in Appendix A. The resultant geometry transformations
for the S0 ! S1 excitation are shown in Fig 5c and d. These
geometry changes have been derived from the LIF excitation
spectra of the 3,5-DCA(–NH2) and 3,5-DCA(–ND2) forms,
respectively. The geometry transformations shown in Fig. 5a
and 5c,d are close to each other. The changes in the benzene
ring geometry shown in Fig. 5a and 5c,d are the same. The
main difference in geometry change predicted by these two
approaches is seen for the substituent groups. The displace-
ment vectors derived from the optimization procedure for
3,5-DCA(ꢀNH2) molecule and 3,5-DCA(ꢀND2) molecule
are shown in Table 6.
The vector Dq representing the geometry change between the
ground state and the excited states and the mode patterns for
the totally symmetric normal vibrations (C2v) identified in the
LIF excitation spectra of 3,5-DCA are shown in Fig. 5. The
displacements of the normal coordinates for the ground state
vibrations between the ground state and the excited state are
given as the products (projections) of the normal coordinates
into vector Dq (see eqn. (A3))
The displacements K for the normal modes are given in
Table 6 for 3,5-DCA(ꢀNH2) and 3,5-DCA(ꢀND2) species.
4. Intensities in the LIF excitation spectra of
3,5-dicyanoaniline
The LIF excitation spectra were measured in narrow ꢄ250–
300 cmꢀ1 overlapping (ꢄ50 cmꢀ1) spectral ranges. The overlap
ranges were used to normalize the spectra. This procedure have
been applied repeatedly to calculate the whole excitation spec-
trum. It is estimated that the relative band intensities are sub-
ject to error from about 5% at low wavenumbers to 20% in the
upper range of the spectrum.
5. Summary
LIF excitation spectra of jet-cooled 3,5-DCA molecules in
native and deuterated forms have been investigated. The
The intensities of the vibrational bands involving the totally
symmetric vibrational modes in the LIF excitation spectrum of
3,5-DCA have been calculated using multimode Franck–
Condon overlap integrals. For this purpose the ab initio results
for the displacement of the normal modes, the vibrational fre-
quencies and Dushinsky rotation matrices have been applied.
The recurrence formulae derived by Ruhoff31,32 have been used
for the intensity calculation. The equations are presented in
Appendix B. Table I (ESI) in column 7 gives the predicted
relative band intensities using the molecular geometries and
the normal modes derived from the HF/6-31++G** calcula-
tion for the ground electronic state and the CIS/6-31++G**
calculation for the lowest excited electronic state of 3,5-
DCA. In this calculation the Dushinsky rotation matrix, dis-
placement parameters K from Table 4 and unscaled vibrational
frequencies have been used. Moreover, all totally symmetric
vibrational modes have been taken into account. From Table
4 it is evident that five vibrational modes with Ki greater than
0.1 give major contribution to the LIF excitation spectrum.
These dominant modes are shown in Fig. 3. The corresponding
mode patterns for the ground state S0 (planar geometry) and
excited S1 state for these dominant modes are drawn. The
results of the modeling of the intensity distributions in the
LIF excitation spectra are presented in Fig. 4.
Table 6 Normal mode displacements for a1 vibrations of the planar
3,5-DCA molecule, from ab initio calculations and from the intensity
distribution
3,5-DCA(–NH2)
3,5-DCA(–ND2)
Kfiit
S0 pl
No
Ki
Kfiit
Ki
2
10
13
14
23
24
28
30
31
33
35
37
39
40
41
ꢀ0.249
0.254
ꢀ0.288
0.313
0.249
ꢀ0.254
0.194
0.277
ꢀ0.353
0.068
ꢀ0.066
0.220
0.137
ꢀ0.177
0.145
0.164
ꢀ0.126
ꢀ0.111
ꢀ0.102
ꢀ0.017
0.110
ꢀ0.229
ꢀ0.201
ꢀ0.053
ꢀ0.007
0.093
0.052
0.079
ꢀ0.043
ꢀ0.124
ꢀ0.040
ꢀ0.015
0.008
ꢀ0.067
ꢀ0.099
ꢀ0.043
ꢀ0.002
0.013
0.045
0.010
0.075
0.044
0.024
0.113
0.023
ꢀ0.031
ꢀ0.007
ꢀ0.000
0.009
ꢀ0.023
0.007
0.000
ꢀ0.004
ꢀ0.004
0.000
ꢀ0.007
0.000
0.007
ꢀ0.031
ꢀ0.039
4104
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107

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Fig. 4 Intensity distribution: peaks of a1 and b1 symmetry. a) from
the LIF spectrum of 3,5-DCA(ꢀNH2); b) modeling: parameters from
ab initio calculations; c) modeling: Ki fitted to reproduce experimental
intensities (see text for details).
has significant intensity. This latter property is a consequence
of large change in the potential function between the A1
ground state (double minimum) and the B2 excited electronic
state (single minimum) for the inversion coordinate of the
amino-group.
Remarkable features of the1B2 LIF excitation spectra of
jet-cooled 3,5-DCA are vibronic activities of these vibrational
modes which involve aromatic ring motion mode No
12 ¼ 6a, No 14 ¼ 7a and No 23 ¼ 1. The similar ring modes
activity have been found earlier for other cyanoanilines: 2,6-
dicyanoaniline21,23 and 3-cyanoaniline. Moreover, it has been
found that the LIF excitation spectrum for deuterated form
of 3,5-DCA(ꢀND2), reveals a line splitting due to Fermi reso-
nance between 6a 446 cmꢀ1 and 16a2 2 ꢂ 222.5 cmꢀ1 modes.
The important difference between ortho- and meta-substi-
tuted cyano derivatives of aniline: 3,5-DCA and 3-cyanoani-
line versus 2,6-DCA and 2-cyanoaniline, is a small intensity
of the band corresponding to the in-plane motion of the cyano
group (C-CN bending, 9a in 3,5-DCA) in meta- substituted
compounds.
It is generally assumed that the predictions concerning
the molecular structure and the vibrational motions in the
excited electronic states reveal distinct deficiency of the CIS
method. This is due to insufficient evaluation of the electron
Fig. 3 Mode patterns of the normal vibrations of planar 3,5-DCA
molecule in its S0 and S1 electronic states. Selected modes give major
contribution to the LIF excitation spectra of 3,5-DCA.
ground state vibrations of 3,5-DCA in crystalline state have
been studied using IR and Raman spectroscopic techniques.
The ab initio HF and CIS calculations of vibrational frequen-
cies have been performed for 2,6-dicyanoaniline in the S0 and
S1 electronic states using the 6-31++G** basis set. The corre-
lations between calculated and measured vibration frequencies
have been proposed for the ground electronic state and the first
excited singlet state S1 (1B2 symmetry). It has been found that
the dominant vibronic bands in the excitation spectra are con-
nected with the modes involving in-plane motions of 3,5-DCA.
The intensity distributions in the LIF excitation spectra of
3,5-DCA were modeled. A qualitative agreement between
measured and calculated intensities have been reached for
the majority of totally symmetric fundamentals.
The assignments of the I20 transition in the LIF excitation
spectra of 3,5-DCA and its deuterated derivatives have been
proposed (904 cmꢀ1/672 cmꢀ1). It has been found that the
frequencies of the I20 bands increase with the number of
cyano substituents in the aniline molecule. The transition I02
involving inversion motion of the amino group in 3,5-DCA
Fig. 5 Geometry changes due to S0 ! S1 excitation.
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107
4105

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upon electronic excitation in the mass weighted Cartesian
coordinates. The components of the displacement vector, Ki ,
represent the projections of the normal coordinates for the
ground state onto the same vector Dq.
The geometric transformation has been performed as
follows:
Since from eqn. (A3) the following relation holds
correlations when the configuration interaction is limited to
singly excited configurations. In particular the electronic exci-
tation energies are considerably overestimated. For the
3,5-DCA molecule, the predicted intensities in LIF excitation
spectra for some totally symmetric vibrations 12 and 23
are underestimated. In conclusion, it seems that the CIS/
6-31++G** calculation underestimates the changes of mole-
cular geometry accompanying the S0(A1) ! S1(B2) electronic
transition in the 3,5-DCA molecule.
K ¼ lg ꢅDq
Present study clearly shows that for the vacuum-isolated
jet-cooled 3,5-DCA molecule where intramolecular hydrogen
bonding is unimportant the substituants of the benzene ring
undergo smaller geometry change than in 2,6-DCA. This
conclusion is well reproduced by the results of the ab initio
calculation shown in Fig. 5a and 5b as well as by the observed
intensities of the in-plane bending vibrations of the cyano
groups in the LIF excitation spectra shown in Fig. 4. and
reported in ref. 23.
we find
Dq ¼ lgT ꢅK
and in Cartesian coordinates we have the following relation
ðA5Þ
gT
cart
Dx ¼ I
ꢅ K
ðA6Þ
contains the
27
Output from the Gaussian98 program
Cartesian normal mode coordinates, which are defined as
pffiffiffiffiffiffi
cart
ik
l
lik were recovered by multiplying l
¼ lik= mk. From these coordinates, components of the
pffiffiffiffiffiffi
cart
ik
by mk. The resultant
lꢅlT matrix was orthogonal and normalized as required.27
Appendix A
Dimensionless displacements for normal mode coordinates
xi were derived using calculated mode frequencies as:
Normal mode coordinates for the ground state (g) and excited
state (e) are linear combinations of mass weighted displace-
ments of the Cartesian coordinates from equilibrium geometry
in the corresponding electronic state, g or e:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Appendix B
xi ¼ Di 2pni=ꢀh.
3N
X
Qgj ðeÞ
¼
ljgkðeÞðqk ꢀ qgðeÞÞ
ðA1Þ
k
Let consider a simple case where the normal coordinates in
the two electronic states are related by the relation (A2b).
k¼1
where Qi are the normal mode coordinates and qk are the mass
weighted Cartesian coordinates of the atoms. In eqn. (A1) the
equilibrium geometries of the molecule are described by the set
of mass weighted Cartesian coordinates {qx} where x ¼ g or e.
Normal coordinates in the two electronic states are related as
follows:
The multidimensional overlap integrals are defined by general
relations.31,32
Z
Iðnq; neÞ ¼ N cgðngÞceðneÞdQe
ðB1Þ
where N ¼ (det J)ꢀ1/2. In more direct form they are given by
Z
Qe ¼ D þ RQg
Qg ¼ K þ JQe
ðA2aÞ
ðA2bÞ
Iðng; neÞ ¼ C exp½ꢀðQgyCgQg þ QeyCeQeÞ=2ꢆ
g1=2
g
ꢂ Hn ðC
Q ÞHn ðCe1=2QeÞdQe
ðB2Þ
g
e
where the components Dj and Kj of the displacement vectors
of the normal modes D and K are given by
In the present calculations of the fluorescence excitation
spectrum we have used diagonal matrices of the corresponding
vibrational frequencies
3N
X
Di ¼
liekðqek ꢀ qgkÞ
0
1
0
1
o1g
0
0
0
0
k¼1
o1e
0
0
0
0
o2g
o2e
B
@
C
A
B
@
C
A
Cg
¼
; Ce ¼
ðB3Þ
and
.
.
.
.
3N
0
0
.
X
whereas the elements Rij and Jjj of the rotation matrices R and
J are given by
0
0
.
Ki ¼
ligkðqek ꢀ qgkÞ
ðA3Þ
The multidimensional Franck–Condon overlap integrals
were calculated using the recursion relations given by Ruhoff
31
for the transitions from the lowest vibrational state. The
k¼1
Franck–Condon factors of the assigned transitions were cal-
culated as squares of the Franck–Condon overlap integrals,
i.e., |I(0;ne)|2. The corresponding overlap integrals are given
in terms of the J, Cg and Ce matrices using the following
definitions:
3N
X
Rij ¼
Jij ¼
liekðlgTÞkj
k¼1
and
3N
X
S ¼ JþCgJ þ Ce; C ¼ 2Ce1=2Sꢀ1Ce1=2 ꢀ 1 and
ligkðleT Þkj
ðA4Þ
X ¼ ꢀ2 Ce1=2Sꢀ1 JþCgK
ðB4Þ
k¼1
respectively. The rotation matrix gives a useful measure of
normal modes correlation, and their mixing upon isotopic
exchange or electronic excitation. This approach is correct
within harmonic approximation for potential energy functions.
The matrix elements of the Dushinsky Rij matrix represent
projections of normal coordinates in the ground electronic
state on normal mode coordinates in the S1 excited state. On
the other hand, the components of the displacement vector
Di represents the projections of the normal coordinates for
the excited state onto the vector Dq: Dqk ¼ qek – qkg, where
k ¼ 1,. . .,3N. The vector Dq describes the geometry change
Relative probabilities for transitions starting in the ground
vibrational level are evaluated as
2
2
jIð0g; neÞj =jIð0g; 0eÞj
ðB5Þ
where the overlap integral I(0g;0e) is given by
!
1=2
1=2
2a detðCgCeÞ
Ið0g; 0; 0; . . . ; 0Þ ¼
detðJSÞ
À
Á
ꢂ exp 21 K ½CgJSꢀ1Jy ꢀ 1ꢆCgK
ðB6Þ
y
4106
Phys. Chem. Chem. Phys., 2003, 5, 4096–4107

View Article Online
and the overlap integrals I(0g;ne) are calculated using the
recursion formula
11 J. M. Hollas, M. R. Howson, T. Ridley and L. Halonen, Chem.
Phys. Lett., 1983, 98, 611.
12 D. Phillips, J. Photochem. Photobiol. A: Chem., 1997, 105, 307.
13 S. Wategaonkar and S. Doraiswamy, J. Chem. Phys., 1996, 105,
1786.
1
Ið0g; 0; . . . ; 0; nm; . . . ; naÞ ¼
½XmðIðnm ꢀ 1Þ
1=2
ð2nmÞ
14 J. August, T. F. Palmer, J. P. Simons, C. Jouvet and W. Rettig,
Chem. Phys. Lett., 1988, 145, 273.
1=2
þ ð2ðnm ꢀ 1ÞÞ
C
Iðnm ꢀ 2Þ
mm
15 J. Herbich, F. P. Salgado, R. P. H. Rettschnick, Z. R. Grabowski
and H. Wo´jtowicz, J. Phys. Chem., 1991, 95, 3491.
16 C. Dedonder-Lardeux, C. Jouvet, S. Martrenchard, D. Solgadi,
J. McCombe, B. D. Howells, T. F. Palmer, A. Subaric-Leitis,
C. Monte, W. Rettig and P. Zimmermann, Chem. Phys. Lett.,
1995, 191, 271.
a
ꢀ
ꢁ
X
þ
1=2ðCmk þ Ckm
Þ
nk
2
k¼mþ1
ꢂ Iðnm ꢀ 1; nk ꢀ 1Þꢆ
ðB7Þ
The eqns. (B5)–(B7) are sufficient for calculation of the
intensity distribution of the vibronic transitions in the LIF
excitation spectra of jet-cooled molecules.
17 A. L. Sobolewski and W. Domcke, Chem. Phys. Lett., 1996,
259, 119.
18 E. M. Gibson, A. C. Jones and D. Phillips, Chem. Phys. Lett.,
1988, 146, 270.
19 H. Yu, E. Joslin, B. Crystall, T. Smith, W. Sinclair and D.
Phillips, J. Phys. Chem., 1993, 97, 8146.
20 R. Howell, E. M. Joslin, A. G. Taylor and D. Phillips, J. Chem.
Soc., Faraday Trans., 1992, 88, 1605.
Acknowledgements
21 P. Kolek, K. Pirowska and J. Najbar, Phys. Chem. Chem. Phys.,
2001, 3, 4874.
22 W. Roth, P. Imhof and K. Kleinermanns, Phys. Chem. Chem.
Phys., 2001, 3, 1806.
´
23 P. Kolek, K. Pirowska, M. Gora, B. Kozik and J. Najbar, Chem.-
Phys., 2002, 285, 55.
This work was supported by grants from the Polish State Com-
mittee for Scientific Research 3T09A 086 16 and KBN/SGI
ORIGIN 2800/UJ/026/1999 and the grants of the Academic
Computer Center. The authors thank Prof. Janusz Sepioł
for his suggestions concerning synthesis of cyanoanilines and
´
Dr A. Birczynska-Wesełucha, Ms M. Jakus and Ms T. Szpyt
24 P. Kolek, P. Gajdek, K. Pirowska and J. Najbar, J. Mol. Struct.,
1999, 511/512, 117; D. T. Mowry, Chem. Rev., 1948, 42,
189–283.
for measuring Raman and IR spectra. We thank also referee
for his valuable comments.
25 D. T. Mowry, Chem. Rev., 1948, 42, 189–283.
26 H. J. Barber and R. Slack, J. Chem. Soc., 1947, 82.
27 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A.
Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A. Montgomery,
Jr., R. E. Stratmann, J. C. Burant, S. Dapprich, J. M. Millam,
A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J. Tomasi,
V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli, C.
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Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.
Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P.
M. W. Gill, B. G. Johnson, W. Chen, M. W. Wong, J. L.
Andres, M. Head-Gordon, E. S. Replogle and J. A. Pople, GAUS-
SIAN 98 (Revision A.9), Gaussian, Inc., Pittsburgh, PA, 1998.
28 J. B. Foresman, M. Head-Gordon, J. A. Pople and M. J. Frisch,
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