Conformational analysis of n-pentyl acetate using microwave spectroscopy

Journal of Molecular Spectroscopy 290 (2013) 24–30

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Conformational analysis of n-pentyl acetate using microwave spectroscopy T. Attig a,⇑, R. Kannengießer a,b, I. Kleiner b, W. Stahl a a

Institute of Physical Chemistry, RWTH Aachen University, Landoltweg 2, D-52074 Aachen, Germany Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR 7583 (CNRS/Univ. Paris Est & Paris Diderot), Université de Paris Est, 61 avenue du Général de Gaulle, F-94010 Créteil cedex, France b

a r t i c l e

i n f o

Article history: Received 23 April 2013 In revised form 28 June 2013 Available online 15 July 2013 Keywords: n-Pentyl acetate MB-FTMW spectroscopy Internal rotation Conformer analysis Gas phase structure

a b s t r a c t The Fourier transform microwave spectrum of n-pentyl acetate (CH3–COO–C5H11) was recorded under molecular beam conditions. The rotational constants and the centrifugal distortion constants of the most abundant and of one less abundant conformer were determined after analyzing the spectrum through comparison with theoretical calculations. The main conformer with C1 symmetry has a strong spectrum. The second observed conformer, which has CS symmetry shows a spectrum with considerably weaker intensities. The quantum chemical calculations of specific conformers were carried out at MP2/6311++G(d,p) level. The values of the experimental rotational constants obtained by the programs XIAM and BELGI were compared to those obtained by ab initio methods. For both conformers torsional barriers of the acetyl methyl group of approximately 100 cm1 were found. Furthermore, the geometry of the main conformer was optimized at different levels of theory. Ó 2013 Elsevier Inc. All rights reserved.

1. Introduction This paper belongs to a series of studies that we recently undertook to determine the structural parameters characterizing various odorant molecules. In particular we are interested in the so-called ‘‘fruit’’ aliphatic esters, such as ethyl acetate [1], methyl propionate [2], ethyl valerate [3], ethyl isovalerate [4], isopropyl acetate [5], isoamyl acetate [6], and n-butyl acetate [7] responsible for the odor of many fruits, flowers like roses, and of the bouquet of young wines. Those molecules are also used as perfuming agent and flavor additives. Some of them are identified in the alarm pheromones of honeybees. For most of these molecules many conformers exist. Still, little was known before our work about the exact geometry of the different conformers and their energy differences. This knowledge could be a crucial step towards the understanding of the odor-structure relationship of molecules in olfactory research as molecular structure is assumed to play a key role in odor recognition. Our objective is thus to provide very precise structural data on these molecules. Due to rather similar energies of different conformers and approximations in quantum chemical calculations, it is often difficult to decide which is the lowest energy conformer. Therefore, molecular beam Fourier transform microwave (MB-FTMW) spectroscopy is used as a validation tool.

⇑ Corresponding author. E-mail address: [email protected] (T. Attig). 0022-2852/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jms.2013.07.001

We want to investigate the influence on the conformational preferences and molecular dynamics when increasing the chain length of the carbon backbone. In this paper we focus on n-pentyl acetate (CH3–COO–C5H11) in order to compare the results with nbutyl acetate (CH3–COO–C4H9) [7]. This could be relevant for the olfactory research since the odors of the fruit esters seem to shift with the hydrocarbon chain length [8]. In Section 2 we describe the quantum chemical calculations carried out to obtain the energies of the conformers and the preferred geometries. The rotational constants from the quantum chemical calculations are essential for making the first assignment of the spectra. In Sections 3 and 4 we present our experimental work. In Section 5 we discuss the results obtained using the two internal rotation codes XIAM and BELGI.

2. Quantum chemical calculations To specify the structure of each conformer of n-pentyl acetate, the notation given in Ref. [6] was utilized. We used the same strategy to study the different conformers as in our previous study on n-butyl acetate [7]. To define the different conformers of n-pentyl acetate the torsional angles hI (C1–O3–C8–C11), hII (O3–C8–C11– C14), hIII (C8–C11–C14–C17), and hIV (C11–C14–C17–C20) (see Fig. 1) were used. For each torsional angle three energy minima are found. These are the Plus synclinal, the antiperiplanar, and the Minus synclinal conformations for the torsional angles around +60°, around 180°, and around 60°, respectively. Due to the fact

T. Attig et al. / Journal of Molecular Spectroscopy 290 (2013) 24–30

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Fig. 1. Structure of conformer aaaa of n-pentyl acetate along with atom labels and torsional angles hI, hII, hIII and hIV. The hydrogen atoms are marked in white, the carbonate atoms in gray, and the oxygen atoms in dark gray.

Fig. 2. Energies calculated with the program gaussian09 [11] for all energetically different conformers of n-pentyl acetate at MP2/6-311++G(d,p) level. The open circles mark the observed conformers. The notation aPaa, PPaa etc. for the conformers are from Ref. [6].

Fig. 3. Rotational constants from the ab initio calculations for all energetically different conformers of n-pentyl acetate at MP2/6-311++G(d,p) level. The rotational constant A is marked in dots, the rotational constant B in triangles and the rotational constant C in squares. The open circles show the values of the experimentally found rotational constants for each assigned conformer.

26

Table 1 Quantum chemical constants and microwave spectroscopic data of n-pentyl acetate obtained with the programs GAUSSIAN09 [11], XIAM [14], BELGI-C1 [17] and BELGI-Cs [16]. Unit

\(i,a) \(i,b) \(i,c) s[c] P[d] N(A/E)[e]

r[f]

GHz GHz GHz kHz kHz kHz kHz kHz kHz kHz GHz uÅ2 GHz cm1 kJ/mol deg deg deg

kHz

3.6086995 0.5878636 0.5361746

158.0690 3.197206

68.746 27.036 74.083

aPaa (Fit1) XIAM

aPaa (Fit2) XIAM

3.6269779(51) 0.5844717(21) 0.53417937(85) 0.0883(20) 2.0384(62) 21.4(10) 0.01391(56) 0.43(13) 58.17(21) 32.15(24) 144.542(63) 3.4964(15) 2741.9(12) 91.459(40) 1.09410(48) 67.3389(04) 27.4097(11) 75.4067(12) 8.370628 15 70/60 14.0

3.627479(91) 0.584827(18) 0.534164(15) 0.117(36) 2.06(11) 62(18) 0.018(10) 0.3(23) 26.1(25) 9.2(19) 158.0791[k] 3.197[k] 2999.46(44) 100.051(15) 1.19688(17) 67.2943(76) 27.395(18) 75.497(20) 8.377855 15 70/60 253.7

aPaa (Fit1) BELGI-C1 3.6278849(68)[h] 0.5857724(64)[h] 0.53460592(49)[h]

144.534369(82) 3.4965801(19) 2742.57(10) 91.4823(34) 67.30160(30) 27.3037(15) 75.64035(68)[i] 8.373072 16 70/60 2.6

aaaa[g] ab initio

aPaa (Fit2) BELGI-C1 3.6279226(72) 0.5858206(68) 0.53462312(53)

[k]

156.5722 3.2277495(19) 3071.81(12) 102.4646(39) 67.30052(32) 75.64819(71) 27.3000(16) 8.657757 16 70/60 2.8

6.4768271 0.4710480 0.4464291

158.0701 3.197183

40.674 49.326 89.965

aaaa XIAM 6.5027420(50) 0.47067669(99) 0.44652323(91) 0.00634(67) 0.200(24)

12.908(67) 6.70(18) 158.0791[k] 3.197[k] 2958.256(76) 98.6768(25) 1.1804(30) 39.8763(28) 50.1237(28) 90.000[k] 8.105549 10 25/25 5.9

aaaa BELGI-CS 6.501856(76)[h] 0.4709752(66)[h] 0.4466513(40)[h] 0.00415(27)

158.3200(65) 3.19212(12) 2998.4(24) 100.0163(79) 39.8933(31) 50.1067(31)[i] 90.000[k] 8.198698 11 25/25 1.7

Note: All constants refer to the principal axis system, for the centrifugal distortion constants Watson’s A reduction and a Ir representations was used. Statistical uncertainties are shown as one standard uncertainty in the last digit. [a] Moment of inertia of the internal rotor. [b] Hindering potential. [c] Reduced barrier s = 4V3/(9F). [d] Number of the fitted parameters. [e] Number of fitted A and E species lines, respectively. [f] Standard deviation of the A/E species fit. [g] MP2/6-311++G(d,p) level. [h] Obtained by transformation from rho axis system to principal axis system. [i] Calculated from \(i,b) = 90°  \(i,a). [k] Fixed value, see text.

T. Attig et al. / Journal of Molecular Spectroscopy 290 (2013) 24–30

A B C DJ DJK DK dJ dK Dpi2J Dpi2 F0 Ic[a] V3[b]

aPaa[g] ab initio

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T. Attig et al. / Journal of Molecular Spectroscopy 290 (2013) 24–30 Table 2 Quantum chemical data obtained for the observed C1 conformer aPaa of n-pentyl acetate at different levels of theory.

HF/6-31G(d,p) HF/6-31+G(d,p) HF/6-31++G(d,p) HF/6-311G(d,p) HF/6-311+G(d,p) HF/6-311++G(d,p) MP2/6-31G(d,p) MP2/6-31+G(d,p) MP2/6-31++G(d,p) MP2/6-311G(d,p) MP2/6-311+G(d,p) MP2/6-311++G(d,p) B3LYP/6-31G(d,p) B3LYP/6-31+G(d,p) B3LYP/6-31++G(d,p) B3LYP/6-311G(d,p) B3LYP/6-311+G(d,p) B3LYP/6-311++G(d,p)

A[a]

dev.[b]

B

dev.

C

dev.

E (Hartree)[c]

ZPE (Hartree)[d]

l (Debye)[e]

3.6682 3.6919 3.6949 3.6698 3.6977 3.6983 3.5615 3.5954 3.5936 3.5493 3.6190 3.6087 3.6004 3.6540 3.6622 3.5929 3.6475 3.6515

1.1 1.8 1.9 1.2 1.9 2.0 1.8 0.9 0.9 2.1 0.2 0.5 0.7 0.7 1.0 0.9 0.6 0.7

0.5854 0.5820 0.5817 0.5855 0.5825 0.5825 0.5933 0.5869 0.5872 0.5944 0.5871 0.5879 0.5786 0.5708 0.5704 0.5799 0.5738 0.5735

0.2 0.4 0.5 0.2 0.3 0.3 1.5 0.4 0.5 1.7 0.4 0.6 1.0 2.3 2.4 0.8 1.8 1.9

0.5343 0.5320 0.5318 0.5347 0.5324 0.5323 0.5395 0.5356 0.5356 0.5403 0.5357 0.5362 0.5280 0.5227 0.5222 0.5291 0.5251 0.5249

0.0 0.4 0.4 0.1 0.3 0.4 1.0 0.3 0.3 1.1 0.3 0.4 1.2 2.2 2.2 1.0 1.7 1.7

423.002767 423.010169 423.010552 423.088997 423.094668 423.094920 422.998890 423.005935 423.006312 423.085216 423.090724 423.090972 425.668168 425.684056 425.684279 425.769634 425.776891 425.777049

422.785122 422.793051 422.793440 422.872688 422.878482 422.878738 424.144568 424.167403 424.169408 424.312702 424.326316 424.327404 425.464249 425.480897 425.481145 425.566947 425.574365 425.574539

2.23 2.39 2.39 2.20 2.35 2.35 2.18 2.36 2.36 2.13 2.28 2.28 2.05 2.29 2.29 2.01 2.23 2.23

Note: All stationary points are energy minima without imaginary frequencies. [a] Rotational constants A, B, C in GHz. [b] The relative deviation in percent with respect to the experimental value. [c] Electronic energies. [d] Sum of electronic and zero-point energies. [e] Dipole moment.

that cis conformers of acetic acid esters are generally higher in energy than trans conformers [4,9,10] only the latter ones were considered. According to this construction principle n-pentyl acetate has 34 = 81 possible trans conformers. Conformer aaaa has CS symmetry. The remaining 80 conformers have C1 symmetry and occur as enantiomeric pairs. In total, 41 energetically different conformers can be generated. Quantum chemical calculations were done for each conformer using the program Gaussian09 [11] to predict the rotational constants, which are necessary to analyze the microwave spectrum. These calculations were done at the MP2/6311++G(d,p) level, which turned out to be a suitable method and basis set for geometry optimizations of alkyl acetic acid esters. Examples for this can be found in former studies on allyl acetate [12], isopropyl acetate [5], ethyl acetate [1], and n-butyl acetate [7]. The optimizations of 40 starting geometries led to stable conformers. The optimization of conformer MPMP was not possible due to sterical hindrance. The obtained energies relative to the conformer with the lowest energy are shown in Fig. 2. The conformer with the lowest energy is aPaa. Conformer aaaa is the twelfth one in the order of increasing energy. The theoretically calculated rotational constants are shown in Fig. 3. All optimized conformers were found to be near prolate tops. The calculated constants of conformer aPaa and aaaa are compared to the experimental constants in Table 1. The rotational constants differ by less than one percent from the experimental ones. To proof the nature of the stationary points frequency calculations were carried out. No imaginary frequencies were obtained. Furthermore, geometry optimizations of conformer aPaa were carried out at different levels of theory to compare the theoretical rotational constants with the experimental data. The results are shown in Table 2. As in the case of n-butyl acetate, the deviation between the experimentally and theoretically found rotational constants is lowest at the MP2/6-311+G(d,p) level. Also the MP2/ 6-311++G(d,p) level which has been used throughout this work is in excellent agreement.

Fig. 4. Broadband Scan of n-pentyl acetate (upper trace) and simulated spectrum for all assigned lines (total, aPaa and aaaa) at a rotational temperature of 1.0 K (lower traces).

calculated energy and strong lines were expected. In the range of 9.75–12.75 GHz strong a- and b-type lines were predicted. Therefore, a broadband scan was recorded in this frequency range using the Aachen MB-FTMW spectrometer [13]. All lines were re-measured in the high-resolution mode were the estimated accuracy is 5 kHz. For the measurements a mixture of approximately 1% n-pentyl acetate in helium was expanded through a pulsed nozzle into the cavity at a stagnation pressure of 100–150 kPa. Due to the low vapor pressure of the substance (5.3 mbar at 20 °C) the ‘‘pipe cleaner’’ method was used as described in Ref. [3]. The n-pentyl acetate was purchased from Merck Schuchardt OHG, Hohenbrunn, Germany and was used without any further purification. 4. Spectral analysis

3. MB-FTMW measurements To make a first prediction of the microwave spectrum the rotational constants of the ab initio calculations were used. We started with the assignment of conformer aPaa, because it has the lowest

In total, 340 lines were measured. All strong lines and most lines of medium intensity were assigned to two conformers. In Fig. 4 the broadband scan is shown in the upper trace. Only lines above a certain intensity limit are drawn. Spectra of the assigned

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T. Attig et al. / Journal of Molecular Spectroscopy 290 (2013) 24–30

conformers aPaa and aaaa simulated at a rotational temperature of 1.0 K are presented in the lower trace. The figure shows the excellent agreement between the calculated and the experimental spectrum. About 150 less intense lines are left over. They probably belong to fractions of spectra related to other unassigned conformers. In the following the assignments of the conformers aPaa and aaaa are described. 4.1. Conformer aPaa (C1 symmetry) We started to assign the spectrum with the lowest energetic conformer. By trial and error, the a-type lines of the A species could immediately be assigned using the program XIAM [14]. In the next step the rotational constants B and C were fixed and the b-type and c-type lines of the A species could be assigned by varying the rotational constant A. This led to a fit of the rotational and quartic centrifugal distortional constants of the A species including 70 lines at a standard deviation of 1.5 kHz. The barrier of internal rotation was expected to be about 100 cm1 as reported for methyl acetate [15] and ethyl acetate [1]. The angles of the internal rotor with respect

to the principal axes were calculated using the optimized ab initio geometries. These values were used in XIAM as an initial guess. In the next step the lines of the E species could be assigned. Finally, a fit with 70 A and 60 E species lines was achieved at a standard deviation of 14.0 kHz. In Table 1 the respective microwave spectroscopic data are shown in column labeled aPaa (Fit1). The rotational constants and the angles of the internal rotor are in a good agreement with the values achieved from the ab initio calculations of conformer aPaa. This confirms that the assigned conformer has the structure aPaa, which is shown in Fig. 5. 4.2. Conformer aaaa (CS symmetry) As seen in Ref. [3] the rotational constants of molecules with CS symmetry can be accurately predicted. For this reason, we decided to assign conformer aaaa. The spectrum was predicted with the rotational constants from the ab initio calculations. By trial and error, the a-type lines of the A species could be assigned. The b-type lines of the A species could be identified by fixing the B and C rotational constants and varying the rotational constant A. Finally, a fit

Fig. 5. Optimized structures of the conformers aPaa and aaaa of n-pentyl acetate at the MP2/6-311++G(d,p) level. On the left the molecules are shown from the side view and on the right from top view. The nuclear coordinates with its numbers are shown in the principal axes system in Å. The hydrogen atoms are marked in white, the carbonate atoms in gray, and the oxygen atoms in dark gray.

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T. Attig et al. / Journal of Molecular Spectroscopy 290 (2013) 24–30

Table 3 Molecular constants for the two observed conformers of n-pentyl acetate obtained by global fits using the programs BELGI-C1 [17] for conformer aPaa and BELGI-Cs [16] for conformer aaaa. Operator[a]

Constant[b]

Unit[c]

P 2a P 2b P 2c

A

GHz

3.2371119(61)

3.2371360(64)

6.479824(77)

B

GHz

0.9737274(59)

0.9737908(63)

0.4930072(50)

C

GHz

0.53742406(36)

0.53743953(40)

{Pa,Pb} {Pa,Pc} P4

Dab Daci DJ DJK

GHz GHz kHz kHz

1.0180101(63) 0.03106623(48)

1.0180227(67) 0.03106563(50)

P 4a

DK

kHz

47.03(47)

46.42(50)

2P 2 ðP 2b  P 2c Þ

dJ

kHz

0.03083(16)

0.03111(17)

P 2 P 2a

fP 2a ; ðP 2b  P 2c Þg (1/2)(1 – cos 3c) (1 – cos 3c)P2

aPaa

aPaa(Fit2)

18.79(17)

18.54(18)

aaaa

0.4466513(40) 0.363850(36) 0.00415(27) 1.364(90) 114.6(82)

dK

kHz

2.098(23)

2.066(24)

ð1  cos 3cÞðP 2b  P 2c Þ

V3 Fv c2

GHz MHz MHz

2742.57(10) 1.6694(15) 0.9906(15)

3071.81(12) 1.7958(17) 1.0608(17)

2998.4(24) 0.4226(62) 0.1513(45)

P 2c

F

GHz

145.57621[d]

157.6908[d]

162.54148[d]

Pa Pc Pa2Pc PaPc{Pa,Pb} P2{Pa,Pb}

q k1 dDELTA DabJ

r[e]

MHz kHz kHz kHz

0.01036205(81) 0.0667(23) 8.95(55) 3.387(28) 2.6

0.01027033(85) 0.0725(26) 8.20(63) 3.347(30) 2.8

0.032279(50)

1.7

Note: Statistical uncertainties are shown as one standard uncertainty in the last digit. [a] All constants refer to a rho-axis system, therefore the inertia tensor is not diagonal and the constants cannot be directly compared to those of a principal axis system. Pa, Pb, Pc are the components of the overall rotation angular momentum, Pc is the angular momentum of the internal rotor rotating around the internal rotor axis by an angle c. {u,v} is the anti-commutator uv + vu. [b] The product of the parameter and operator from a given row yields the term actually used in the vibration–rotation–torsion Hamiltonian, except for F, q, and A, which occur in the Hamiltonian in the form FðP c  qP a Þ2  AP 2a . [c] Values of the parameters from the present fit. [d] Values fixed to XIAM values. [e] Standard deviation of the A/E species fit.

with 25 A and 25 E species lines was achieved with a standard deviation of 5.9 kHz. The spectroscopic data of conformer aaaa is shown in Table 1. The rotational constants and the angles of the internal rotor are in good agreement with the values obtained from the ab initio calculations. The structure of conformer aaaa is shown in Fig. 5. In order to confirm the assignment and the results obtained with XIAM both conformers have also been fitted with the internal rotation program BELGI. The latter code exists in two versions to account for both, molecules with a mirror plane (CS) or such without any symmetry elements (C1). Both codes have been described in detail in the literature [14,16,17] and are available at the PROSPE website [18]. Comparisons of XIAM with ERHAM and XIAM with SPFIT can be found in Refs. [19,20], respectively. The XIAM results for the rotational constants A, B, C as well as for the internal rotation parameters V3, F, and q were used as initial values in BELGI. An initial estimation of the value for the Dab parameter (multiplying the PaPb + PbPa operator in the rho axis system RAM) was derived from the nuclear coordinates theoretically calculated by Gaussian09 [11] at the MP2/6-311++G(d,p) level. The fits yielded excellent standard deviations of 2.6 kHz and 1.7 kHz for aPaa and aaaa using BELGI-C1 and BELGI-Cs, respectively. The results of both fits are given in Table 3 in the RAM system. In order to compare the results to XIAM the values of the main parameters were transformed into the principal axis system (PAM) and are shown in Table 1. 5. Comparison of XIAM and BELGI The results obtained with XIAM and BELGI (see Table 1) show excellent compliance throughout the whole record. For conformer aPaa the torsional barrier height of the acetyl methyl group is 91.459(40) cm1 and 91.4823(34) cm1 with XIAM and BELGI, respectively, which is a difference of less than one percent. The torsional barrier of the n-pentyl methyl group is estimated to be 1000 cm1. Test calculations have shown that this barrier is too high to produce observable internal rotation splittings. The fitted

Ic has a value of 3.4964(15) uÅ2 for XIAM and 3.4965801(19) uÅ2 for BELGI. Both values differ by about less than one percent. The angles between the direction of the top and the principal axes differ by 0.007–0.14° for conformer aPaa. The largest difference was obtained for the angle \(i,c).The moment of inertia Ic is too high in both fits with XIAM and BELGI and just has to be considered as an effective value. A more realistic Ic can be calculated from the ab initio optimized nuclear coordinates of the three hydrogen atoms of the acetyl methyl group. This way, a value of 3.197 uÅ2 is obtained at the MP2/6-311++G(d,p) level. In a second XIAM fit 2 we decided to fix F0 = 2Ih c to 158.0791 GHz, which corresponds to the ab initio value of Ic. The results are shown in Table 1 labeled aPaa (Fit2). Since the torsional barrier height V3 is correlated with the internal rotation constant F0 when only ground torsional state data is available the change of Ic also causes a change of V3. Here V3 of 100.051(15) cm1 is obtained [5]. The rotational constants and the angles of the internal rotor did not change much. A drawback of this treatment is that the standard deviation becomes worse with a value of 253.7 kHz. It reveals that nonrigidity effects are not taken into account in our model. It was also possible to produce a comparable fit with BELGI by fixing F to 157.691 GHz labeled aPaa (Fit2). In this case V3 was found to be 102.4646(39) cm1. This is a torsional barrier height which is typical for small alkyl acetic acid esters. It may be compared with the molecules methyl acetate [21], ethyl acetate [1], and n-butyl acetate [7] which have barriers of 101.740(30) cm1, 99.57(11) cm1, and 99.66(36) cm1 obtained with the BELGI code, respectively. For conformer aaaa a torsional barrier of 98.6768(25) cm1 was obtained with XIAM and 100.0163(79) cm1 with BELGI. This is a difference of 1.34% between both methods. For Ic a value of 3.19212(12) uÅ2 was obtained from BELGI, whereas the XIAM value was fixed to the calculated one of 3.197 uÅ2. The values differ by 0.15%. In the BELGI fit the aaaa conformer yielded thus a value for Ic which is much closer to its expected theoretical value. The internal rotation angles \(i,a) and \(i,b) obtained with XIAM and BELGI are in very close agreement. The angle \(i,c) was fixed to 90° due to CS symmetry.

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T. Attig et al. / Journal of Molecular Spectroscopy 290 (2013) 24–30

It is interesting, that the lowest energy conformers of n-pentyl acetate aPaa can be obtained by adding a terminal methyl group in antiperiplanar configuration to the lowest energy conformer of n-butyl acetate aPa. In a same manner the CS n-pentyl acetate conformer aaaa can be generated from the CS n-butyl acetate conformer aaa. Comparing the Ic values of n-butyl acetate and n-pentyl acetate the same tendency was obtained for the C1 conformers. The Ic values were higher than the theoretical values. In the case of the CS conformers the n-pentyl acetate value of Ic is more realistic than in the case of n-butyl acetate, where Ic was 3.29 uÅ2. Nevertheless, in both cases the fits of n-butyl acetate and n-pentyl acetate yielded better values of Ic for the CS conformers than for the C1 conformers. Generally, C1 conformers are more difficult to fit since more terms in the Hamiltonian are necessary to describe the less symmetric system. As pointed out in our isoamyl acetate study [6] the discrepancy of the methyl inertial moment seems to indicate that the nonrigidity effects are particularly important for larger molecular systems. In the case of isoamyl acetate these non-rigidity effects were assumed to come from small amplitude vibrations. By consequence the vibrations also affect the moment of inertia of the acetate methyl group and this effect is not taken into account in our fits containing only ground state torsional transitions within vt = 0. For the present n-pentyl acetate C1 conformer similar nonrigidy effects can also occur. For the Cs conformer the structural influence seems to be less important. Acknowledgments We thank the PROCOPE project of the Deutscher Akademischer Austauschdienst, the NRW Forschungsschule ‘‘BrenaRo’’ for funding, and the Center for Computing and Communication of the RWTH Aachen University for free computer time. Appendix A. Supplementary material Supplementary data for this article are available on ScienceDirect (www.sciencedirect.com) and as part of the Ohio State University Molecular Spectroscopy Archives (http://library.osu.edu/sites/ msa/jmsa_hp.htm). Frequency lists of the observed conformers of n-pentyl acetate, the nuclear coordinates in the inertial axis system of all conformers optimized at the MP2/6-311++G(d,p) level, and a

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