isomeric identification of methylated naphthalenes using gas phase infrared spectroscopy

ORIGINAL PAPER

Isomeric identification of methylated naphthalenes using gas phaseinfrared spectroscopy

S Chakraborty, P Das and P K Das*

Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

Received: 22 June 2011 / Accepted: 07 March 2012 / Published online: 18 April 2012

Abstract: We report gas phase mid-infrared spectra of 1- and 2- methyl naphthalenes at 0.2 cm-1 resolution. Assignment

of observed bands have been made using scaled quantum mechanical (SQM) calculations where the force fields rather the

frequencies are scaled to find a close fit between observed and calculated bands. The structure of the molecules has been

optimized using B3LYP level of theory in conjunction with standard 6-311G** basis set to obtain the harmonic fre-

quencies. Using the force constants in Cartesian coordinates from the Gaussian output, scaled force field calculations are

carried out using a modified version of the UMAT program in the QCPE package. Potential energy distributions of the

normal modes obtained from such calculations helped us assign the observed bands and identify the unique features of the

spectra of 1- and 2-MNs which are important for their isomeric identification.

Keywords: Infrared spectra; FT-IR spectrometer; Vibrational analysis; Density functional theory

PACS Nos.: 33.20.Ea; 07.57.-c; 33.20.Tp; 31.15.E-

1. Introduction

Naphthalene and its methylated isomers belong to the class

of polycyclic aromatic hydrocarbons (PAHs). Methylnaph-

thalenes (MNs) are formed in the environment due to

incomplete combustion of diesel, cigarette smoke, etc.

[1–3]. Diehl et al. [4] determined the amount of aromatic

hydrocarbons in gasolines by gas chromatography/fourier

transform infrared spectroscopy (GC/FT-IR) technique and

found that the amounts of 1- and 2-MN are *0.08 ± 0.01

and 0.18 ± 0.01 %, respectively in the total amount of

aromatic hydrocarbons present in gasoline. Chiu et al. [5]

performed structural characterization of polycyclic aromatic

compounds from a mixture of 20 authentic compounds that

have been identified in various combustion extracts

(including 1- and 2-MN) by GC/MS and GC/FT-IR tech-

niques. They recorded the IR spectra at 8 cm-1 resolution

and concluded that FT-IR spectroscopy provides unambig-

uous differentiation between the structural isomers of MN.

The isomeric identification has been done based on quali-

tative assignment of aromatic C–H out-of-plane bending

vibrations appearing in the region of 740–830 cm-1.

Recently Iavicoli et al. [6] have determined the amounts of

airborne PAH contents at an airport by gas-chromatography/

mass spectrometry (GC/MS) and found that the concentra-

tions of 1- and 2-MNs are high and vary in the range

24–35,000 and 64–28,500 ng/m3, respectively. We have

found that the NIST reported vapor phase infrared spectra of

1- and 2-MN were recorded at low resolution (2–3 cm-1)

and several peaks are not well resolved [7]. In this work we

have recorded the gas phase IR spectra of 1- and 2-MN with

high resolution and performed scaled force field calculations

to assign the spectra unambiguously. The aim of our work is

two-fold: (i) to record the gas phase IR spectra of 1- and

2-MN and assign the vibrational bands and (ii) to look for

unique regions of the spectra based on which fingerprinting

of the MN isomers can be done.

2. Experimental

MNs used in this study are 1-methyl naphthalene (95 %)

and 2-methyl naphthalene (97 %) were bought from

Aldrich, USA. The compounds were converted into gas

phase by heating and then were introduced into a multi-

pass long-path gas cell (Model 7.2-V, REFLEX Analyti-

cal Corporation, path length 6.0 m) by mixing with UHP

� 2012 IACS

*Corresponding author, E-mail: [email protected]

Indian J Phys (March 2012) 86(3):209–218

DOI 10.1007/s12648-012-0042-1

argon as a carrier gas. All spectra were recorded in a

vertex-70 (Bruker Optik) FT-IR spectrometer. Details of

the experimental set-up have been described elsewhere

[8]. The long-path cell was wrapped with a heating tape

and a feedback heat sensor was kept on the cell to read

the temperature. The experimental temperature was set to

80 and 70 �C for 1- and 2-MN, respectively. The gas cell

was kept inside a wooden box, whose inside portion was

covered with silicon sheet. This was done to protect the

spectrometer from the heat. During the experiments, the

spectrometer was purged constantly with UHP nitrogen

gas to keep moisture and carbon dioxide away. We have

recorded the spectra keeping the entrance slit width at

2 mm. The spectral resolution was set at 0.2 cm-1. The

integrated band areaR

log I0=I dm under a band was cal-

culated by using the OPUS software provided by Bruker

Optik. The experimental intensity was calculated using

the relation [9]

A ¼ 2:303

Zlog I0=Ið Þ dm=pl ð1Þ

where A is the intensity in km mol-1, p is the partial

pressure (in atm) of the compound inside the gas cell and

l is the path length in cm. Since the compound is seeded

with argon, it is difficult to determine the partial pressure of

the compound inside the cell. From the DFT calculated

intensity and observed band area, the estimated vapor

pressures (pi’s) over all the bands of MN were obtained.

The vapor pressures from all the observed bands were then

averaged to get p. The same value of the partial pressure

was then plugged into Eq. (1) to obtain the experimental

band intensities. The details of the intensity calculation

have been discussed in our earlier papers [8, 10].

3. Computational method

The structures of 1- and 2-MN were optimized at the B3LYP/

6-311G** level of theory using Gaussian 03 [11] in a IBM-64

cluster. Harmonic frequencies and intensities of MNs were

calculated with the optimized structure. Figure 1 shows the

optimized structures of MNs. In our early work on dimethyl

naphthalenes [8], the harmonic frequencies from Gaussian

out-put were scaled with two different scaling factors for two

different regions of the spectrum as was suggested by Bau-

schlicher Jr. et al. [12] and a large discrepancy between the

theoretical and the experimental frequencies was observed. To

remove this discrepancy we have later adapted the scaled

quantum mechanical (SQM) procedure for obtaining experi-

mental frequencies as proposed by Pulay et al. [13] using the

modified version of the UMAT program in the QCPE package

[14]. In this method force fields are scaled instead of fre-

quencies by minimizing the fitting error between calculation

and experiment. The details of the force field calculation have

been discussed in ref [10]. The Cartesian force constant matrix

from the Gaussian output was transformed to the nonredun-

dant local coordinate matrix of the MNs [15]. All the 57

nonredundant local coordinates of the MNs are shown in

Table 1. The experimental frequencies have been fitted with

the calculated frequencies through 100 iterations using algo-

rithm given in [16]. Generally, the error in fitting is within a

few wavenumbers, in this method.

4. Results and discussion

The calculated spectra along with the experimental are

shown in Fig. 2. For the theoretical spectra the FWHM was

Fig. 1 Optimized geometry of 1- and 2-MN at the B3LYP level in

conjunction with 6-311G** basis set. The internal coordinates have

been shown in the picture. f01 defines one of the angle coordinates of

the methyl group. Twist coordinates, s’s are designated as the same

way like C–C bond coordinates, (bi) i=1–3 represents H–C–H angle

coordinates

210 S. Chakraborty et al.

Table 1 List of nonredundant local coordinates for 1- and 2-MN

Compounds Nonredundant local coordinatesa

1-MN S1 - S11 = (Ri) i=1–11 (aromatic C–C stretching)

S12 = R0012 (aromatic-methyl, C–C stretching)

S13 - S19 = (ri) i=1–7 (aromatic C–H stretching)

S20 - S22 = (r0i) i=1–3 (methyl C–H stretching)

S23 = 2-1/2(A1 - A2) (b01) (aromatic C-methyl in-plane bending)

S24 - S30 = 2-1/2 (A3 - A4)…(A15 - A16), (bi) i=2–8 (aromatic C–H in-plane bending)

S31;34 ¼ 6�1=2 a1 � a2 þ a3 � a4 þ a5 � a6ð Þ d1; d4ð ÞS32;35 ¼ 12�1=2 2a1 � a2 � a3 þ 2a4 � a5 � a6ð Þ d2; d5ð ÞS33�36 ¼ 0:5 a2 � a3 þ a5 � a6ð Þ d3; d6ð Þ

9>=

>;

dið Þi¼1�6

represent

ring deformations

S37 = 6-1/2 (f01 ? f02 ? f03 - b1 - b2 - b3) (methyl sym. def.) (da)

S38 ¼ 6�1=2 2f01 � f02 � f03� �

methyl antisym: def:ð ÞS39 ¼ 2�1=2 f02 � f03

� �methyl antisym: def:ð Þ

S40 ¼ 6�1=2 2b1 � b2 � b3ð Þ rocking methylð ÞqCH3

9>>=

>>;ds

S41 = 2-1/2 (b2 - b3) (rocking methyl) qCH3

S42 = c01 (aromatic C-methyl out-of-plane bending)

S43 - S49 = (c0i) i=2–8 (aromatic C–H out-of- plane bending)

S50;53 ¼ 6�1=2 s1 � s2 þ s3 � s4 þ s5 � s6ð Þ s1; s4ð ÞS51;54 ¼ 1=2 s1 � s3 þ s4 � s6ð Þ s2; s5ð ÞS52;55 ¼ 2�1=2 �s1 þ 2s2 � s3 � s4 þ 2s5 � s6ð Þ s3; s6ð Þ

9>=

>;

sið Þi¼1�6

represent

ring torsions

S56 = 2-1/2(s10-9-21-1 - s7-9-21-19) (s07) s ring

S57 = s10–11 (aromatic—methyl twist) (s008)

2-MN S1 - S11 = (Ri) i=1–11 (aromatic C–C stretching)

S13 = R0012 (aromatic-methyl, C–C stretching)

S12, S14 - S19 = (ri) i=1–7 (aromatic C–H stretching)

S20 - S22 = (r0i) i=1–3 (methyl C–H stretching)

S24 = 2-1/2 (A3 - A4) (b02) (aromatic C-methyl, in-plane bending)

S23, S 25–30 = 2-1/2 (A1 - A2)��(A15 - A16), (bi)i=1,3–8 (aromatic C–H in-plane bending)

S31;34 ¼ 6�1=2 a1 � a2 þ a3 � a4 þ a5 � a6ð Þ d1; d4ð ÞS32;35 ¼ 12�1=2 2a1 � a2 � a3 þ 2a4 � a5 � a6ð Þ d2; d5ð ÞS33�36 ¼ 0:5 a2 � a3 þ a5 � a6ð Þ d3; d6ð Þ

9>=

>;

dið Þi¼1�6

represent

ring deformations

S37 = 6-1/2 (f01 ? f02 ? f03 - b1 - b2 - b3) (methyl sym. def.) (da)

S38 ¼ 6�1=2 2f01 � f02 � f03� �

methyl antisym: def:ð ÞS39 ¼ 2�1=2 f02 � f03

� �methyl antisym: def:ð Þ

S40 ¼ 6�1=2 2b1 � b2 � b3ð Þ rocking methylð ÞqCH3

9>>=

>>;ds

S41 = 2-1/2 (b2 - b3) (rocking methyl) qCH3

S42 = c01 (aromatic C-methyl out-of-plane bending)

S43 - S49 = (c0i) i=2–8 (aromatic C–H out-of-plane bending)

S50;53 ¼ 6�1=2 s1 � s2 þ s3 � s4 þ s5 � s6ð Þ s1; s4ð ÞS51;54 ¼ 1=2 s1 � s3 þ s4 � s6ð Þ s2; s5ð ÞS52;55 ¼ 2�1=2 �s1 þ 2s2 � s3 � s4 þ 2s5 � s6ð Þ s3; s6ð Þ

9>=

>;

sið Þi¼1�6

represent

ring torsions

S56 = 2-1/2(s10–9–21–1 - s7–9–21–19) (s07) s ring

S57 = s10–11 (aromatic—methyl twist) (s008)

a See Fig. 1 for the internal coordinates

Isomeric identification of methylated 211

assumed to be 15 cm-1 which is close to what is observed

experimentally. Tables 2 and 3 list the fundamental har-

monic, observed, and force field fitted vibrational fre-

quencies with the corresponding intensities for 1- and

2-MN, respectively. From experimental spectra we

found * 30 bands although, each MN has 57 fundamen-

tals which correspond to irreducible representation

(38A0 ? 19A00) under the Cs point group. This is a signif-

icant improvement over the NIST reported spectra of the

MNs where only 21 bands are seen in the 1-MN and 22

bands in the 2-MN spectra. Observed band assignments

have been done based on the fitted frequencies and

potential energy distributions (PEDs) obtained from the

SQM calculation. The entire spectrum has been divided

into three regions. In the following section we have

discussed about the details of the band assignments in

each region and isomeric identification of MNs in the gas

phase.

4.1. Spectra in the region 3,200–2,800 cm-1

This region contains two characteristic sets of bands cor-

responding to aromatic C–H and methyl C–H stretches. We

have found six bands at 3108.9, 3076.9, 3062.2, 3048.6,

3015.6, and 2981.8 cm-1 for 1-MN and five bands at

3095.6, 3062.9, 3024.5, 2979.9, and 2963.4 cm-1 for

2-MN. These bands are due to aromatic C–H stretching

vibrations. The corresponding fitted frequencies are at

3108.9, 3076.9, 3062.2, 3048.6, 3015.6, and 2981.8 cm-1

for 1-MN and 3095.0, 3062.9, 3024.5, 2979.9, and

2963.5 cm-1 for 2-MN, respectively. In 1-MN DFT cal-

culation predicts seven bands; however, we observed six

bands. One band at 3003.0 cm-1 is not observed perhaps

due to low intensity. The next three bands in this region

observed at 2956.1, 2916.5, and 2876.7 cm-1 for 1-MN

and 2956.8, 2934.7, and 2877.6 cm-1 for 2-MN have been

assigned to antisymmetric and symmetric C–H stretching

0.0

0.2

0.4

0.6

0.0

0.2

0.4

1-MN Theoretical

Rel

ativ

e In

tens

ity

Wavenumber (cm -1 )

Experimental

3108

.9

3076

.9

3062

.230

48.6

3015

.6 2981

.8

2956

.1

2916

.5

2876

.7

2745

.3

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.3

0.6

0.9

1-MNTheoretical

853.

6

Rel

ativ

e In

tens

ity

Wavenumber (cm -1)

Experimental

1603

.2

1514

.214

72.8

1444

.7

1399

.7

1267

.7

1213

.011

67.7

1077

.2

1022

.8

565.

6

1053

.1

978.

8

788.

9

770.

972

7.4 53

1.3

1942

.619

26.3

0.0

0.3

0.6

0.9

0.0

0.4

0.8

2-MN

Rel

ativ

e In

tens

ity

Theoretical

Wavenumber(cm-1)

Experimetal

1644

.616

04.6

1513

.214

68.1

1440

.813

83.6

1364

.8

1271

.6

1211

.5

1134

.6

1014

.5

699.

2

952.

2

849.

5

737.

9

812.

1

620.

9

1942

.219

20.6 77

7.0

1173

.5

884.

5

0.0

0.2

0.4

0.6

0.8

1.0

3200 3100 3000 2900 2800 2700 2000 1800 1600 1400 1200 1000 800 600

2000 1800 1600 1400 1200 1000 800 6003200 3100 3000 2900 2800 2700

0.0

0.3

0.6

2-MN

Rel

ativ

e In

tens

ity

Theoretical

3095

.6

Wavenumber(cm-1)

Experimetal3062.9

3024

.5

2979

.9

2956

.8 2934

.7

2877

.6

2744

.5

Fig. 2 Normalized experimental and theoretical gas phase IR spectra of 1- and 2-MN. The spectra are normalized w. r. t. the intense band found

in the region of 785–815 cm-1


Table 2 Calculated harmonic frequencies (cm-1) and intensities (km mol-1) at the B3LYP/6-311G** level, observed frequencies (cm-1) and

intensities (km mol-1), fitted frequencies (cm-1) and PED (%) of 1-MN

Sym B3LYP/6-311G** Observed Force field

No Harm. Int. Freq.a Int. (Rel. Int.)b Fitted PED

A0

1 3193.8 15.807 3108.9 2.16 (0.044) 3108.9 r4 (97)

2 3183.1 25.775 3076.9 26.18 (0.544) 3076.9 r6 (90)

3 3182.2 34.316 3062.2 9.76 (0.203) 3062.2 r2 (95)

4 3167.5 12.869 3048.6 16.52 (0.344) 3048.6 r5 (89)

5 3163.7 19.376 3015.6 7.95 (0.165) 3015.6 r3 (93)

6 3157.2 1.117 (3003.0) 3003.0 r7 (95)

7 3155.3 2.495 2981.8 9.47 (0.197) 2981.8 r1 (97)

8 3104.7 19.117 2956.1 16.94 (0.352) 2956.4 r20(84) ? r1

0 (14)

9 3019.8 29.355 2876.7 11.28 (0.234) 2874.8 r10 (86) ? r2

0 (14)

10 1664.8 0.276 (1625.0) 1628.4 R8 (12) ? R10 (10)

11 1641.6 9.211 1603.2 4.96 (0.103) 1602.3 R2 (19) ? R4 (18) ? R8(12) ? R10 (11)

12 1620.4 1.322 (1590.0) 1586.8 R6 (15) ? R3 (12)

13 1548.4 8.438 1514.2 6.27 (0.130) 1517.0 R9 (20)

14 1505.3 7.124 1472.8 2.28 (0.047) 1474.1 dsCH3 (63)

15 1494.6 0.370 (1465.0) 1464.8 b2 (15) ? b5 (12) ? b10 (10)

16 1465.8 2.803 1444.7 6.13 (0.127) 1440.5 dsCH3 (20) ? b7 (17)

17 1425.7 12.718 1399.7 8.73 (0.181) 1398.0 b4 (15) ? b6 (12)

18 1416.2 1.479 1383.3 1.41 (0.029) 1383.2 d6 (85)

19 1391.9 0.307 (1365.0) 1365.8 R6 (30)

20 1376.0 0.963 (1349.0) 1347.7 R2 (13) ? R10 (12)

21 1290.4 3.361 1267.7 2.47 (0.051) 1268.1 b8 (23) ? b4 (19) ? b7 (17) ? R11 (10)

22 1263.7 0.236 (1239.0) 1239.7 b10(28)

23 1236.6 1.802 1213.0 0.96 (0.020) 1212.6 R5 (23) ? R7 (15) ? R11 (13) ? b6 (11)

24 1189.4 1.958 1167.9 1.38 (0.028) 1168.6 b2 (40) ? b3 (15)

25 1185.3 0.724 (1162.0) 1162.1 b5 (37) ? b4 (17) ? R8 (15)

26 1168.5 0.754 (1146.0) 1147.3 b6 (24) ? R10 (19) ? b7 (17)

27 1098.8 3.526 1077.2 5.45 (0.113) 1076.9 R3 (40) ? b10 (12)

28 1078.8 0.041 (1058.0) 1057.7 d3 (30) ? R1200 (17) ? d8CH3 (14)

29 1045.4 5.824 1022.8 7.80 (0.162) 1022.4 R9 (54)

30 994.9 1.844 (975.0) 977.7 dsCH3 (28) ? d3 (16) ? b8 (15)

31 871.3 0.244 (854.0) 854.6 d3 (17) ? b8 (15) ? R1200 (14) ? d1 (11) ? R6 (11)

32 803.9 0.797 (792.0) 790.4 b8(21) ? d2 (17) ? d3(17) ? d5 (16)

33 712.4 1.569 (698.0) 698.4 d4 (36) ? R6 (13) ? R1200(10)

34 577.4 1.491 565.6 1.61 (0.033) 565.3 d4 (38) ? d1 (18)

35 520.2 0.197 (510.0) 510.2 d5 (24) ? d2 (18) ? d1 (18) ? d4 (10)

36 483.9 0.878 (474.0) 474.1 d1 (28) ? d2 (24) ? d5 (20)

37 442.7 1.319 (434.0) 433.8 r30 (44) ? d2 (15) ? d5 (13)

38 278.5 0.445 (273.0) 273.1 r30 (55) ? d5 (16) ? d2 (14)

A00

39 3065.7 20.366 2916.5 8.66 (0.180) 2918.2 r10 (100)

40 1487.7 7.900 1465.6 2.01 (0.041) 1459.4 qCH3 (92)

41 1061.7 1.590 1053.1 5.15 (0.107) 1048.4 daCH3 (50) ? dsCH3 (17) ? qCH3 (10)

42 996.2 0.510 978.8 4.20 (0.087) 978.6 c5(32) ? c6 (31) ? c7 (12) ? c4 (11) ? s3 (10)

43 983.5 0.700 (964.0) 963.3 c2(48) ? c10 (17) ? c3 (17)

44 964.0 0.873 (945.0) 944.1 c7 (29) ? c4 (25) ? c6 (18) ? c5 (16)


vibrations of the methyl group. The corresponding force

field fitted frequencies are calculated at 2956.4, 2918.2,

and 2874.8 cm-1 for 1-MN and 2955.0, 2930.4, and

2883.3 cm-1 for 2-MN, respectively. In the aromatic and

methyl C–H stretching region the mean deviation between

the observed band intensity and the calculated one is large

and is found to be *11 km mol-1. This large deviation

may be due to overlapping of bands or to the occurrence of

Fermi resonances.

4.2. Spectra in the region 2,800–1,800 cm-1

From Fig. 2 we note that there are a few bands in this region

in the observed spectra but they are absent in the calculated

spectra for both the MNs. They are nonfundamental bands

since they do not correspond to any of the fundamental cal-

culated frequencies of the MNs. They are either overtone or

combination bands [10]. The band observed in the spectra of

1-MN at 2745.3 cm-1 having an absolute intensity of

0.484 km mol-1 is assigned as the first overtone of the

aromatic C–H in-plane bending vibration, m17 by comparing

with the force field fitted frequency at 2795.4 cm-1 (see

Table 4). In 2-MN one nonfundamental band found at

2744.5 cm-1 having an absolute intensity of 0.462

km mol-1 has been assigned as the first overtone of the

aromatic ring deformation, m18 with the help of the calculated

force field fitted frequency at 2766.8 cm-1. A low intensity

band observed at 1942.6 cm-1 in 1-MN and at 1942.2 cm-1

in 2-MN is assigned as a combination band of (m29 ? m42) in

1-MN and (m28 ? m43) in 2-MN, respectively. These two

bands are correlated with the force field fitted frequency at

2001.1 cm-1 in 1-MN and at 1968.7 cm-1 in 2-MN. The

next nonfundamental low intensity vibrational band

observed at 1926.3 and at 1920.6 cm-1 for 1- and 2-MN,

respectively, is assigned as a combination of fundamentals

(m17 ? m51) in 1-MN and (m26 ? m47) in 2-MN. Bands

observed in this region deviate by *50 cm-1 from their

corresponding force field fitted frequency.

4.3. Spectra in the region 1,800–500 cm-1

Methyl substitution leads to many intense bands in this

region of the spectra, which were not prominent in naph-

thalene. In the spectra of 1-MN there is one band at

1603.2 cm-1 whereas in 2-MN there are two clearly visible

bands of moderate intensity at 1644.6 and 1604.6 cm-1.

From Table 2 it is seen that in 1-MN the corresponding force

field fitted frequency is calculated at 1602.3 cm-1. From

PEDs it has been found that for this vibration more than two

aromatic C–C bond motions are involved. In 2-MN, the

aromatic C–C stretching vibration appears at the fitted fre-

quency of 1609.2 cm-1 where only two different aromatic

C–C bond motions are involved. The other peak calculated at

1643.5 cm-1 in 2-MN consists of a mixture of aromatic C–C

stretching and aromatic ring deformation. This band found at

1628.4 cm-1 in the calculation in 1-MN is not seen in the

experimental spectra either due to very low intensity which is

Table 2 continued


No Harm. Int. Freq.a Int. (Rel. Int.)b Fitted PED

45 913.3 0.001 (895.0) 895.2 c10 (46) ? c3 (33)

46 873.2 1.018 853.6 1.48 (0.030) 855.6 c7 (25) ? c4 (23) ? c8 (11) ? s3 (11)

47 808.1 55.563 788.9 48.04 (1.000) 790.6 c3 (22) ? c8 (17) ? c5 (14) ? s3 (14) ? c2 (10)

48 790.2 41.776 770.9 21.19 (0.441) 773.1 s3 (27) ? c8 (26) ? c6 (18)

49 744.1 3.062 727.4 0.59 (0.012) 727.6 c2 (17) ? c5 (17) ? c3 (16) ? c7 (15) ? c4(11)

50 638.4 0.009 (626.0) 624.9 s3 (36) ? c8 (30)

51 545.6 6.371 531.3 6.47 (0.134) 532.1 s2 (22) ? qCH3 (20) ? s6 (20) ? s1(11)

52 479.6 0.035 (470.0) 469.6 s1 (28) ? s4 (28) ? s2 (10)

53 419.0 4.802 (410.0) 410.3 s5 (49) ? s4 (11) ? qCH3 (10)

54 248.8 2.828 (244.0) 244.2 s2 (36) ? s4 (19) ? qCH3 (14) ? c10 (10)

55 183.8 2.621 (180.0) 180.1 s70 (50) ? s6 (16) ? qCH3 (12)

56 166.5 0.310 (163.0) 162.7 s70 (25) ? s2 (22) ? s5 (21) ? s6 (19)

57 132.6 0.191 (130.0) 130.1 s1 (44) ? s4 (24)

a Since the fitting algorithm requires all the experimental frequencies in the SQM calculation, the number in the parentheses is introduced as a

good guess for 1-MN and do not have any other significance. RMS error is 1.4 cm-1 for the fitting of the experimental frequencies with the

calculated frequenciesb Relative intensity has been calculated w. r. t. the highest intense band found at 788.9 cm-1 having an absolute intensity of 48.04 km mol-1


Table 3 Calculated harmonic frequencies (in cm-1) and intensities (in km mol-1) at the B3LYP/6-311G** level, observed frequencies (in

cm-1) and intensities (in km mol-1), fitted frequencies (in cm-1) and PED (%) of 2-MN


No Harm Int. Freq. Int. (Rel. Int.)a Fitted PED

A0

1 3185.3 25.505 3095.6 b 3095.0 r6 (92)

2 3172.4 45.568 3062.9 61.93 (1.771) 3062.9 r5 (88)

3 3170.8 14.763 (3059.0) 3059.0 r3 (98)

4 3159.5 1.485 3024.5 11.76 (0.336) 3024.5 r4 (93)

5 3154.3 10.741 (3010.0) 3010.0 r7 (97)

6 3151.8 10.853 2979.9 14.94 (0.427) 2979.9 r2 (97)

7 3151.5 5.004 2963.4 0.82 (0.023) 2963.5 r1 (92)

8 3105.2 17.231 2956.8 0.51 (0.014) 2955.0 r20 (72) ? r1

0(20)

9 3021.1 38.027 2877.6 12.38 (0.354) 2883.3 r10(79) ? r2

0 (21)

10 1673.5 9.919 1644.6 7.21 (0.206) 1643.5 R4 (19) ? R2 (18) ? R1 (10) ? d2(10)

11 1647.1 9.205 1604.6 7.73 (0.221) 1609.2 R8 (20) ? R10 (19)

12 1612.0 0.079 (1580.0) 1579.6 R6 (16) ? R2 (12) ? R3 (10)

13 1545.1 12.568 1513.2 13.05 (0.373) 1515.6 R9 (16) ? b6 (11)

14 1505.0 1.934 (1476.0) 1476.2 dsCH3 (60)

15 1497.7 3.757 1468.1 2.51 (0.071) 1469.2 b5 (15) ? b4 (10)

16 1464.1 4.288 1440.8 3.36 (0.096) 1439.2 b6 (16) ? dsCH3 (14)

17 1416.0 0.528 (1388.0) 1389.0 R4 (17) ? R6 (15) ? R2 (14) ? d6(12)

18 1404.5 3.267 1383.6 2.63 (0.075) 1383.4 d6 (72)

19 1395.5 0.411 1364.8 1.30 (0.037) 1361.7 r30 (17) ? b2

0 (14)

20 1384.6 2.664 (1357.0) 1354.0 R8 (20) ? R10 (15) ? R6 (12)

21 1289.3 5.222 1271.6 3.40 (0.097) 1271.2 b8 (22) ? b7 (21) ?b4 (17) ? R7(11)

22 1276.5 0.324 1231.3 1.08 (0.031) 1234.4 r30 (20) ? b3 (20) ? R1(15) ? d3 (15)

23 1233.5 0.937 1211.5 1.22 (0.034) 1211.7 R5 (20) ? R11 (15) ? b5 (10) ? b6 (10)

24 1194.1 1.927 1173.5 1.70 (0.048) 1173.3 r30 (26) ? R12

00 (17)

25 1179.6 0.955 (1160.0) 1159.9 b5 (17) ? b6 (17) ? b7 (12)

26 1171.8 1.532 1134.6 5.30 (0.151) 1133.7 b3 (41) ? b20 (16) ? R4 (12)

27 1148.5 2.607 (1126.0) 1125.3 b5 (13) ? R8 (12)

28 1041.9 2.074 1014.5 6.31 (0.180) 1014.1 R9 (53)

29 1023.9 7.355 (1004.0) 1004.2 dsCH3 (46) ? daCH3 (15)

30 967.7 0.897 (949.0) 949.1 b8 (31) ? d3 (21) ? R3 (16)

31 896.0 0.151 (878.0) 877.6 d3 (43) ? R1200 (11)

32 781.4 0.147 (766.0) 766.3 R6 (34) ? R5 (12) ? R11(12) ? d1(10)

33 711.8 0.390 699.2 1.30 (0.037) 699.3 b8 (25) ? R1200 (19) ? d1 (16) ? d5(14)

34 636.6 2.540 620.9 2.97 (0.085) 621.2 d4 (53) ? d1 (30)

35 527.7 0.064 (517.0) 516.5 d5 (35) ? d2 (24) ? d4 (13)

36 453.3 0.050 (444.0) 443.9 d1 (26)? d5 (16)

37 412.7 1.509 (404.0) 404.1 d2 (50) ? b1 (28)

38 264.1 0.824 (259.0) 259.0 b1 (55) ? d5 (16)

A00

39 3068.1 21.084 2934.7 23.74 (0.679) 2930.4 r10 (100)

40 1486.1 6.885 1453.1 2.26 (0.064) 1453.1 qCH3 (92)

41 1063.2 3.568 1040.2 2.88 (0.082) 1039.8 daCH3 (51) ? dsCH3 (17)

42 994.8 0.003 (975.0) 974.7 c6 (36) ? c7(29) ? c5(14) ? s3(10)

43 975.4 1.253 952.2 5.31 (0.152) 954.6 c3 (36) ? c20 (28) ? c4 (11) ? c5(11)

44 961.3 0.969 (942.0) 943.1 c4 (28) ? c5 (21) ? c7 (16) ? c20 (12)


apparent from calculation or it is masked by the more intense

band observed at 1603.2 cm-1. The observed band at

1644.6 cm-1 in 2-MN can be used for the isomeric identi-

fication of MN in a mixture since it is clearly resolved from

the adjacent band at 1604.6 cm-1 and is not seen in 1-MN. A

low intensity band observed at 1514.2 and 1513.2 cm-1 for

1- and 2-MN, respectively, is assigned as another aromatic

C–C stretching vibration in 1-MN whereas in 2-MN this

band is a mixture of aromatic C–C stretching and aromatic

C–H in-plane bending vibrations. A band observed at 1465.6

and 1453.1 cm-1 for 1- and 2-MN, respectively, is of low

intensity and belongs to A00 symmetry. From calculation this

band at 1459.4 cm-1 in 1-MN and 1453.1 cm-1 in 2-MN is

assigned as a rocking vibration of the methyl group. A low

intensity band is observed in the spectra of MNs at

1444.7 cm-1 for 1-MN and 1440.8 cm-1 for 2-MN. This

band is assigned to a mixture of methyl antisymmetric

deformation and aromatic C–H in-plane bending motion

from the calculated PEDs.

A unique band seen at 1399.7 cm-1 in the recorded

spectra of 1-MN, is assigned to an aromatic C–H in-plane

bending vibration by comparing with the fitted frequency at

1398.0 cm-1 and its PED. This band can be used as a

marker for isomeric identification of 1-MN. There is no

band in the neighborhood of 1,390 cm-1 in 2-MN. A very

low intensity band is seen at 1383.3 and 1383.6 cm-1 for

1- and 2-MN, respectively, which is correlated with the

aromatic ring deformation vibration from calculated fre-

quencies at 1383.2 cm-1 in 1-MN and 1383.4 cm-1 in

Table 3 continued


No Harm Int. Freq. Int. (Rel. Int.)a Fitted PED

45 903.6 3.356 884.5 5.38 (0.154) 885.4 qCH3 (43) ? c8 (14)

46 864.6 14.635 849.5 9.91 (0.283) 849.4 qCH3 (34) ? c7 (18) ? c6 (17) ? c4 (16)

47 826.7 43.520 812.1 34.96 (1.000) 808.4 c20 (32) ? c3 (32) ? c5 (13)

48 783.7 1.884 777.0 0.81 (0.023) 772.7 s3(45) ? c8 (33)

49 753.0 26.090 737.9 21.27 (0.608) 738.0 c5 (23) ? c4 (19) ? c6 (19) ?c7 (15)

50 642.3 1.372 (629.0) 631.9 c8 (35) ? s3 (28) ? c1 (14)

51 516.1 0.060 (506.0) 506.0 s1 (21) ? s5 (20) ? c1 (18) ? s4 (13)

52 486.0 19.449 (476.0) 475.7 s6 (26) ? s2 (25) ? s5 (15) ? s4 (13)

53 402.2 0.196 (394.0) 393.4 s2 (25) ? s1 (18) ? s5 (18) ? s4 (16)

54 280.5 0.313 (275.0) 274.9 s5 (35) ? c1 (25) ? c8 (12)

55 180.1 0.773 (176.0) 176.0 s1 (38) ? s4 (31) ?s6 (13)

56 119.1 1.079 (116.0) 116.2 s2 (32) ? s6 (17) ? s5 (12)

57 87.7 0.547 (86.0) 86.0 s70 (73) ? c1 (13)

RMS error is 1.8 cm-1 for the fitting of the experimental frequencies with calculated frequenciesa Relative intensity has been calculated w. r. t. the highest intense band found at 812.1 cm-1 having an absolute intensity of 34.96 km mol-1

b The band is not resolved well except for a small shoulder appearing with the 3062.9 cm-1 band. Therefore, individual band intensity is not

reported

Table 4 Experimental and force field fitted nonfundamental bands in

MNs

Molecule Observed

nonfundamental

bands

Force field fitted

nonfundamental bands

Freq. Int. Overtone Combination

1-MN 2745.3 0.484 2795.4 (2m17)

1942.6 0.282 2001.1 (m29 ? m42)

1926.3 1.527 1929.8 (m17 ? m51)

2-MN 2744.5 0.462 2766.8 (m18)

1942.2 1.011 1968.7 (m28 ? m43)

1920.6 0.703 1942.1 (m26 ? m47)

Frequencies are in cm-1 and corresponding absolute intensities in

km mol-1

Table 5 A few characteristic and unique bands of MNs

Mode of vibration 1-MN 2-MN

Aromatic C–H stretching 3076.9 (0.544) 3062.9 (1.771)

Aromatic C–C stretching 1077.2 (0.113) 1644.6 (0.206)

Aromatic C–H in-plane bending 1399.7 (0.181) 1134.6 (0.151)

Aromatic C–H out-of-plane 978.8 (0.087) 952.2 (0.152)

Bending 788.9 (1.000) 812.1 (1.000)

Frequency values are in cm-1. In parenthesis, relative intensities are

given


2-MN. A band appearing at 1267.7 and 1271.6 cm-1 for

1-and 2-MN, respectively, has been assigned to a mixture

of aromatic in-plane bending and aromatic C–C stretching

vibrations based on PEDs. A band found at 1134.6 cm-1 in

2-MN is of low intensity and assigned as aromatic C–H in-

plane bending vibration by comparing with the force field

fitted frequency at 1133.7 cm-1. This band is unique and

absent in 1-MN which perhaps is helpful for its identifi-

cation. Another unique band seen at 1077.2 cm-1 in 1-MN

corresponds to the fitted frequency at 1076.9 cm-1. This

band is assigned to an aromatic C–C stretching vibration

where only one C–C bond is involved (Table 2). Next to

this a low intensity band observed at 1053.1 and

1040.2 cm-1 for 1- and 2-MN, respectively, has been

assigned to a mixture of methyl antisymmetric and sym-

metric deformations based on the PEDs of the fitted band

at 1048.4 cm-1 in 1-MN and 1039.8 cm-1 in 2-MN,

respectively. For both the 1- and 2-MNs a low intensity

band observed at 1022.8 and 1014.5 cm-1, respectively, is

correlated with the force field fitted frequency at 1022.4

and 1014.1 cm-1. This band has been assigned as a pure

aromatic C–C stretching vibration. One low intensity band

seen at 978.8 cm-1 in 1-MN is unique and is assigned as

aromatic C–H out-of-plane bending vibration after com-

paring with the fitted frequency at 978.6 cm-1. The same

band is shifted to 952.2 cm-1 in 2-MN. Two highly intense

bands are seen at 788.9 and 770.9 cm-1 in 1-MN and at

812.1 and 737.9 cm-1 in 2-MN which correspond to aro-

matic C–H out-of-plane bending vibrations. The respective

fitted frequencies are 790.6 and 773.1 cm-1 in 1-MN and

808.4 and 738.0 cm-1 in 2-MN. This band is clearly dis-

tinct for different isomers of MN. The position 1 on

naphthalene is very reactive in comparison to position 2.

This leads to a drastic change in the vibrational fre-

quencies on changing the substitution, and in fact this

happened. Two bands seen at 565.6 and 531.3 cm-1 in

1-MN are assigned to aromatic in-plane ring deformation

and out-of-plane vibration whereas in 2-MN two bands

observed at 699.2 and 620.2 cm-1 correspond to aromatic

C–H out-of-plane and aromatic in-plane ring deformation

vibrations. In naphthalene p-electron localization occurs

and all the C–C bonds are not of equal length [17]. A

methyl substitution will exert more influence on the C–C

bond distances depending on where it is substituted. The

influence is greater when it is in 1 position than when it is

in position 2. This influence has been seen in the aromatic

C–H out-of-plane bending frequencies of 1- and 2-MN. In

1-MN, both the frequency and intensity are less compared

to those in 2-MN. The mean deviation between the cal-

culated and experimental intensities in the non C–H

stretching region is much better compared to that in the

aromatic and methyl C–H stretching region and found to

be *3 km mol-1.

4.4. Identification of isomers

Many aromatic C–H stretching and aromatic C–H out-of-

plane bending vibrations have been identified in the experi-

mental spectra of MNs. Out of these we have chosen a few

either intense or unique bands, as listed in Table 5, for the

isomeric identification. An intense aromatic C–H stretch has

been identified at 3076.9 and 3062.9 cm-1, respectively in 1-

and 2-MN. This band is separated by 14 cm-1 from one

isomer to the other. Therefore, this band can be used for

isomeric identification of MN in a complex mixture. One

unique band observed at 1644.6 cm-1 in 2-MN and another

at 1077.2 cm-1 in 1-MN. These bands are unique to those

isomers and are not observed in the other isomer. Another

unique band seen in the experimental spectra of 1- and 2-MN

at 1399.7 and 1134.6 cm-1, respectively, for aromatic C–H

in-plane bending vibration. This band is of low intensity and

clearly visible in the recorded spectra of MNs. Two sets of

bands have been identified for aromatic C–H out-of-plane

bending vibrations at 788.9 and 978.8 cm-1 in 1-MN and

812.1 and 952.2 cm-1 in 2-MN. The first set of bands is

highly intense and the second set is of low intensity. These

bands are clearly distinguishable for different isomers of

MN. Therefore, spectral bands in the 1,800–500 cm-1

region will be helpful for isomeric identification of MNs in a

complex mixture.

5. Conclusion

The gas phase IR spectra of 1- and 2-MN at 0.2 cm-1 have

been reported in this study which is a clear improvement on

the NIST reported spectra of these molecules. The fitted fre-

quencies and their corresponding PEDs of different mode of

vibrations obtained from SQM calculation helped us do the

unambiguous assignment of the observed bands. By looking at

the highly intense aromatic C–H out-of-plane bending vibra-

tions in the region 1,800–500 cm-1 and at the aromatic C–H

stretching vibrations in the region 3,200–2,800 cm-1, it is

possible to distinguish between the MNs. The isomeric iden-

tification through infrared spectra of these two compounds as

suggested here will perhaps be relevant in the field of the

environmental and atmospheric chemistry.

Acknowledgments The FT-IR spectrometer is supported by the

FIST program of the Department of Science and Technology, Govt.

of India. We thank CSIR, Govt. of India for supporting this research.

Many helpful discussions with E. Arunan and S. Manogaran are

gratefully acknowledged.

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isomeric identification of methylated naphthalenes using gas phase infrared spectroscopy

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