- Email: [email protected]
Ring puckering splitting and structure of indan
Journal of Molecular Spectroscopy 316 (2015) 45–48
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Ring puckering splitting and structure of indan Laura B. Favero a, Weixing Li b, Giancarlo Spadini b, Walther Caminati b,⇑ a b
Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Sezione di Bologna CNR, via Gobetti 101, I-40129 Bologna, Italy Dipartimento di Chimica ‘‘G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy
a r t i c l e
i n f o
Article history: Received 2 July 2015 In revised form 30 July 2015 Available online 8 August 2015 Keywords: Structure Large amplitude motions Rotational spectroscopy Supersonic expansion Hydrocarbons
a b s t r a c t The ground state ring puckering splitting of the cyclopentane ring of indan has been precisely determined (DE01 = 22.364(1) MHz) by measuring the very weak lc-type transitions with pulse jet Fourier transform microwave spectroscopy. In addition, the rotational spectra of all 13C monosubstituted isotopologues have been assigned and measured in natural abundance, leading to an accurate structure of the heavy atoms frame. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Molecules formed by the condensation of a six-membered aromatic ring and a five-membered saturated ring are characterized by three large amplitude motions: ring puckering, flapping and ring twisting. Information on these large amplitude motions have been obtained by microwave (MW), far infrared (FIR), vibrationally or rotationally resolved laser induced fluorescence (LIF) and from a combination of these techniques for a variety of this class of condensed-ring compounds [1–18]. It has been shown in several cases that an unambiguous determination of the barrier to ring puckering can be obtained only when the MW measurements of the ring puckering tunneling splitting in the ground state are available [3,4,6,10,15]. Indan can be considered as the prototype model for this kind of molecular systems. Five spectroscopic investigations are reported in the literature, related to the above mentioned large amplitude motions [1–3,18–20]. They concern the following techniques: FIR [1], vibronic [2,18], MW [3], one-color, resonance enhanced twophoton ionization spectra (R2PI) and dispersed fluorescence (DF) [19], and two-color resonantly enhanced multiphoton ionization and zero-kinetic-energy photoelectron spectroscopy [20], respectively. The rotational spectrum of indan has been reported a long time ago, measured with conventional MW spectroscopy [3]. Due to the very low value of the lc dipole moment component, it was not possible to measure the interstate lc-type transitions, which supply ⇑ Corresponding author. Fax: +39 051 2099456. E-mail address: [email protected] (W. Caminati). http://dx.doi.org/10.1016/j.jms.2015.08.001 0022-2852/Ó 2015 Elsevier Inc. All rights reserved.
directly the value of the vibrational tunneling splitting. This splitting was indirectly estimated by MW-RF double resonance experiments, observing the intensity change of some rotational transitions while changing the value of a radiofrequency signal applied to the Stark electrodes. Nowadays, it is feasible, with pulsed Fourier transform MW spectroscopy, to measure even quite weak transitions. For this reason, we decided to measure the lctype transitions of the most abundant isotopic species (normal) of indan, and in order to determine the molecular structure, the rotational spectra of its 13C-mono substituted isotopologues in natural abundance. The molecular shape of indan, inclusive of the principal axes and of the atom numbering used through the text, is shown in Fig. 1. 2. Experimental methods A commercial sample of indan 98% (Aldrich) was used without further purification. The spectra of the mono-substituted 13C isotopologues were analyzed in natural abundance. The rotational spectra have been measured with pulsed jet Fourier-transform microwave (FT-MW) spectroscopy [21], in a coaxially oriented beam-resonator arrangement-(COBRA)-type [22]. The spectrometer, working in the 6–18.5 GHz frequency region, is described elsewhere [23]. Helium, as carrier gas, was passed over indan at 60 °C temperature, at a backing pressure of about 0.2 MPa, and expanded through the pulsed valve (General valve, series 9, nozzle diameter 0.5 mm) into the Fabry–Perot cavity to about 1 103 Pa. The spectral line position was determined after Fourier transformation of the 8 k data point time domain signal, recorded at intervals of 100 ns. Each rotational transition is
46
L.B. Favero et al. / Journal of Molecular Spectroscopy 316 (2015) 45–48 Table 1 Spectroscopic parameters of the parent species of indan. Single values are common to both states v = 0 and v = 1. v=0
v=1 3531.0444(7)a
A (MHz) B (MHz) C (MHz) DJ (Hz) DJK (Hz) DK (kHz) d1 (Hz) d2 (kHz) DE01 (MHz) r/rexpb Nc
1498.3739(1) 1082.8714(1)
1498.3750(1) 1082.8724(1) 32.2(7) 52(4) 0.6(1) 7.1(4) 2.9(3) 22.364(1) 1.3 242
a
Fig. 1. Molecular shape, principal axes and atom numbering of indan.
Error in parentheses in units of the last digit. Root-mean-square deviation of the fit, referred to an estimated measure errors of 3 kHz and of 50 kHz for the transition frequencies of this investigation and for those of Ref. [3], respectively. c Number of lines in the fit (81 from the present work and 161 from Ref. [3]). b
split by Doppler effect, enhanced by the coaxial arrangement of the supersonic jet and resonator axes in the COBRA-FTMW spectrometer. The rest frequency is calculated as the arithmetic mean of the Doppler components. The estimated accuracy of frequency measurements is better than 3 kHz and lines separated by more than 7 kHz are resolvable. 3. Results and discussion 3.1. Rotational spectra The la-spectrum of the most abundant species has been easily identified, based on the rotational constants determined in the previous absorption room temperature MW investigation [3]. Although being intrastate transitions, they displayed small splittings, due to the difference of the rotational constants of the two tunneling states originated by the above mentioned ring puckering motion. To detect the much weaker lc-type lines we needed the maximum MW pulse power of the spectrometer and to warm up the sample to 60 °C before of the supersonic expansion, in order to increase its concentration in the beam. All these interstate transitions were split into two evenly separated (by ca 44.7 MHz, the double of the ring puckering splitting) component lines. We have been able to measure 67 la-type and 14 lc-type transitions, which have been fitted together with the rotational frequency reported in Ref. [3]. In the fit, the appropriate weight was given to the two sets of measurements. All transitions have been fitted simultaneously with a coupled Hamiltonian using the Pickett set of programs [24]. We used the following expressions:
H ¼ Ri HRi þ HCD þ DE01 ; with i ¼ 0; 1
or 13C8, 13C6 or 13C7), while the fifth isotopologue (13C2) is only 1%. A portion of the spectrum displaying the 312 211 transitions of the parent and 13C isotopologues in natural abundance is shown in Fig. 2. All rotational transitions are also reported in the Supplementary Material. They have been fitted in the same way as the normal species, supplying the spectroscopic constants of Table 2, where also the number of measured transitions is given. Due to the low number of experimental frequencies, the centrifugal distortion parameters have been fixed to the values of the normal species.
ð1Þ
where represents the rotational Hamiltonian for the state i. HCD accounts for the centrifugal distortion corrections, corresponding to the Ir-representation of Watson’s ‘‘S” reduced Hamiltonian [25], assumed to be the same for both states. DE01 is the energy difference between the v = 0 and v = 1 tunneling states. The spectroscopic constants obtained are reported in Table 1. A general improvement on the uncertainties of the experimental parameters has been achieved, with respect to Ref. [3]. In addition, the DE01 (=22.364(1) MHz) has been precisely determined from the fit, the rotational constants have been obtained separately for the two tunneling states, and all quartic centrifugal distortion constants have been fitted. We investigated then the rotational spectra of the five possible 13 C-monosubstituted isotopologues, in natural abundance. Four of these isotopologues have a concentration of ca. 2% of that of the most abundant species, because concern the substitution of one of two equivalent carbon atoms (13C1 or 13C3, 13C4 or 13C9, 13C5 HRi
Fig. 2. Portion of the spectrum with the 312 isotopologues in natural abundance.
211 transitions of the parent and 13C
Table 2 Experimental spectroscopic parameters of the 13C isotopologues (measured in natural abundance) of indan. Centrifugal distortion constants and the tunneling splitting have been fixed to the values of the parent species.
13
C1 C2 13 C4 13 C5 13 C6
(or
13
C3)
(or (or (or
13
C9) C8) C7)
13
a b c
13 13
A (MHz)
B (MHz)
C (MHz)
rb (kHz)
Nc
3494.71(1)a 3529.16(1) 3519.11(1) 3483.37(1) 3519.39(1)
1486.9640(3) 1472.0361(3) 1498.2566(3) 1493.8816(3) 1476.9880(3)
1073.5820(2) 1069.2427(2) 1081.7008(2) 1076.0163(2) 1070.5935(2)
0.9 0.6 0.6 0.8 0.9
9 9 9 9 9
Standard error in parentheses in units of the last digit. Standard deviation of the fit. Number of fitted transitions.
47
L.B. Favero et al. / Journal of Molecular Spectroscopy 316 (2015) 45–48 Table 3 Substitution coordinates (rs) of the carbon atoms in the principal axes system of parent indan. a (Å)
C2 C1(or C4(or C5(or C6(or a b
C3) C9) C8) C7)
b (Å)
c (Å)
rs
re
rs
re
rs
re
±2.4460(6)a ±1.6008(9) ±0.15(1) ±1.005(2) ±2.2123(7)
2.4366 1.6112 0.1925 1.0100 2.2169
0.07(2)ib ±1.223(1) ±0.696(2) ±1.407(1) ±0.697(2)
0.0 1.2204 0.7026 1.4128 0.7016
±0.288(5) ±0.14(1) ±0.06(3) ±0.01(13)ib ±0.05(3)
0.3136 0.1483 0.0634 0.0072 0.0578
Errors in parenthesis are expressed in units of the last digit. Imaginary value.
3.2. Structure and ab initio calculations
3.3. Barrier to ring puckering
From the rotational constants of the various isotopologues it has been possible to obtain the substitution cartesian coordinates of the carbon atoms by applying the Kraitchmann method [26] and using Costain’s uncertainties [27]. The results obtained results are shown in Table 3. We also calculated the MP2/6-311++G(d,p) [28] geometry of indan, which is reported in Table 4. The puckering angle s (180° – XYC2) is visualized in Fig. 1. X and Y represent the intermediate points between the C4–C9 and C1–C3 carbon atoms, respectively. In Table 4, the rs structural parameters of the C atoms are also shown. In turn, the principal axes ab initio cartesian coordinates of the C atoms are reported in Table 3 for comparison with the experimental values. The ab initio calculations also provided the rotational and first order centrifugal distortion constants, as well as the values of the la and lc components of the electric dipole moment (lb is zero by symmetry). All these parameters are shown in Table 5. They are in agreement with the experimental observations. The experimental rotational constants could be reproduced with the ab initio geometry within 1 or 2 MHz, after changing the value of s from 33.2 to 35°.
The ab initio calculations also provide the energy of the planar conformation of the main frame of indan, which difference with respect to the global minimum energy corresponds to the barrier to ring puckering. The value B2 = 733 cm1 has been deduced. This is quite similar to the value obtained by a MP2/6-31++G(d,p) calculation (B2 = 670 cm1) [19]. Both values are quite higher than the experimental values obtained by MW measurements (B2 = 434 cm1) [3] or by laser-induced fluorescence experiments (B2 = 488 cm1) [18]. As to the B2 value obtained with the previous MW analysis, we are satisfied with is because we confirmed the reliability of the indirectly obtained DE01 splitting (22.3 MHz, now precisely determined to be 22.364(1) MHz). Some experimental and theoretical values of the potential energy function are summarized in Table 6, while a graphical comparison of the MW and of the MP2/6-311++G(d,p) potential energy surfaces is given in Fig. 3.
Table 6 Experimental and theoretical values of the potential energy function of the ring puckering motion in indan.
Table 4 Theoretical (re, MP2/6-311++G(d,p)) geometry and rs coordinates of the C atoms of indan. Bond lengths (Å) re/rs C1–C2 C1–C9 C4–C5 C4–C9 C5–C6 C6–C7 C1–H10 C1–H11 C2–H12 C2–H13 C5–H16 C6–H17
re/rs
1.5440/1.547(7) 1.5126/1.543(9) 1.3978/1.359(9) 1.4051/1.392(3) 1.4024/1.402(5) 1.4032/1.394(3) 1.0947 1.0991 1.0959 1.0941 1.0881 1.0870
C1C2C3 C2C3C4 C3C4C9 C4C5C6 C5C6C7 C8C9C4 H10C1C9 H11C1C9 H12C2Y H13C2Y H16C5C4 H17C6C5
Significant dihedral angles (°)a s = 32.3/30(3) H10C1–C9C4 = 141.8 a
B2 (cm1)
s0 (°)
Valence angles (°) a
104.4/104.5(3) 102.4/103.2(8) 110.0/110.0(7) 119.0/118(1) 120.5/120.4(3) 120.5/121.5(8) 113.2 110.0 131.6 141.5 120.7 119.8
Experiments
Theory
MW
LIF (Ref. [18])
MP2/6-31++G(d,p) (Ref. [19])
MP2/6-311++G(d,p). This work
434 34.0
488 8.5a
670 30.8
733 32.3
Deduced from the value x1 = 0.14 Å at the energy minima.
H11C1–C9C4 = 97.6
All dihedral angles of the aromatic part very close to 0 or 180°.
Table 5 MP2/6-311++G** spectroscopic parameters of indan. A (MHz) B (MHz) C (MHz) la/D lc/D
3519 1496 1082 0.6 0.036
DJ (Hz) DJK (Hz) DK (Hz) d1 (Hz) d2 (Hz)
33.1 42.2 234 8.5 1.4 Fig. 3. MP2/6-311++G** and MW experimental shapes of the puckering B2 barrier.
48
L.B. Favero et al. / Journal of Molecular Spectroscopy 316 (2015) 45–48
4. Conclusions
References
With the present investigation we precisely measured the DE01 splitting, 22.364(1) MHz, related to the puckering of the metkylenic apex of the five membered ring of indan, and show that the previously indirectly measured splitting (DE01 = 22.3) was a reliable value. We also assigned the rotational spectra of the five possible 13C monosubstituted isotopologues. The obtained rs geometrical parameters are in agreement with the ab initio values. It is interesting to outline that the theoretical methods do not supply satisfactory values for the parameters concerning the puckering motion. The B2 barrier appear overestimated by about 60%, while the equilibrium value of the s angle is underestimated. Its r0 value (referred to the vibrational ground state) is determined, indeed, to be 35°, larger than the ab initio value, (re, 32.3°). Actually, having the vibrational ground state wavefunction a considerable density also in the region in between the two minima, we should have re > r0.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgments We acknowledge Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. W.L. thanks the China Scholarships Council (CSC) for a scholarship.
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). Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.jms.2015.08.001.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
T.L. Smithson, J.A. Duckett, H. Wieser, J. Phys. Chem. 88 (1984) 1102. K.H. Hassan, J.M. Hollas, J. Mol. Spectrosc. 147 (1991) 100. W. Caminati, D. Damiani, G. Corbelli, L.B. Favero, Mol. Phys. 75 (1992) 857. W. Caminati, L.B. Favero, B. Velino, F. Zerbetto, Mol. Phys. 78 (1993) 1561. K.H. Hassan, J.M. Hollas, Chem. Phys. Lett. 169 (1990) 17. W. Caminati, D. Damiani, L.B. Favero, Mol. Phys. 79 (1993) 699. S. Sakurai, N. Meinander, J. Laane, J. Chem. Phys. 108 (1998) 3537. J.A. Duckett, T.C. Smithson, H. Wieser, Chem. Phys. Lett. 64 (1979) 261. K.H. Hassan, J.M. Hollas, Chem. Phys. Lett. 157 (1989) 183. W. Caminati, S. Melandri, G. Corbelli, B.L. Favero, R. Meyer, Mol. Phys. 80 (1993) 1297. S. Sakurai, N. Meinander, K. Morris, J. Laane, J. Am. Chem. Soc. 121 (1999) 5056. J. Laane, E. Bondoc, S. Sakurai, K. Morris, N. Meinander, J. Choo, J. Am. Chem. Soc 122 (2000) 2628. G. Pietraperzia, A. Zoppi, M. Becucci, E. Droghetti, E. Castellucci, Chem. Phys. Lett. 385 (2004) 304. Z. Kisiel, L. Pszczólkowski, G. Pietraperzia, M. Becucci, W. Caminati, R. Meyer, PCCP 6 (2004) 5469. P. Ottaviani, W. Caminati, Chem. Phys. Lett. 405 (2005) 68. J.A. Yang, K. Okuyama, K. Morris, J. Laane, J. Phys. Chem. A 109 (2005) 8290. E. Bondoc, T. Klots, J. Laane, J. Phys. Chem. A 104 (2000) 275. Z. Arp, N. Meinander, J. Choo, J. Laane, J. Chem. Phys. 116 (2002) 6648. A. Das, K.K. Mahato, S.S. Panja, T. Chakraborty, J. Chem. Phys. 119 (2003) 2523. Y. He, W. Kong, J. Chem. Phys. 122 (2005) 244302. T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. J.-U. Grabow, W. Stahl, H. Dreizler, Rev. Sci. Instrum. 67 (1996) 4072. W. Caminati, A. Millemaggi, J.L. Alonso, A. Lesarri, J.C. Lopez, S. Mata, Chem. Phys. Lett. 392 (2004) 1. H.M. Pickett, J. Mol. Spectrosc. 148 (1991) 371. J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, Elsevier, New York/Amsterdam, 1977, pp. 1–89. 6. J. Kraitchman, Am. J. Phys. 21 (1953) 17. C.C. Costain, G.P. Srivastava, J. Chem. Phys. 35 (1961) 1903. Gaussian 09, Revision D.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian Inc, Wallingford CT, 2013.
Contents lists available at ScienceDirect
Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms
Ring puckering splitting and structure of indan Laura B. Favero a, Weixing Li b, Giancarlo Spadini b, Walther Caminati b,⇑ a b
Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Sezione di Bologna CNR, via Gobetti 101, I-40129 Bologna, Italy Dipartimento di Chimica ‘‘G. Ciamician” dell’Università, Via Selmi 2, I-40126 Bologna, Italy
a r t i c l e
i n f o
Article history: Received 2 July 2015 In revised form 30 July 2015 Available online 8 August 2015 Keywords: Structure Large amplitude motions Rotational spectroscopy Supersonic expansion Hydrocarbons
a b s t r a c t The ground state ring puckering splitting of the cyclopentane ring of indan has been precisely determined (DE01 = 22.364(1) MHz) by measuring the very weak lc-type transitions with pulse jet Fourier transform microwave spectroscopy. In addition, the rotational spectra of all 13C monosubstituted isotopologues have been assigned and measured in natural abundance, leading to an accurate structure of the heavy atoms frame. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction Molecules formed by the condensation of a six-membered aromatic ring and a five-membered saturated ring are characterized by three large amplitude motions: ring puckering, flapping and ring twisting. Information on these large amplitude motions have been obtained by microwave (MW), far infrared (FIR), vibrationally or rotationally resolved laser induced fluorescence (LIF) and from a combination of these techniques for a variety of this class of condensed-ring compounds [1–18]. It has been shown in several cases that an unambiguous determination of the barrier to ring puckering can be obtained only when the MW measurements of the ring puckering tunneling splitting in the ground state are available [3,4,6,10,15]. Indan can be considered as the prototype model for this kind of molecular systems. Five spectroscopic investigations are reported in the literature, related to the above mentioned large amplitude motions [1–3,18–20]. They concern the following techniques: FIR [1], vibronic [2,18], MW [3], one-color, resonance enhanced twophoton ionization spectra (R2PI) and dispersed fluorescence (DF) [19], and two-color resonantly enhanced multiphoton ionization and zero-kinetic-energy photoelectron spectroscopy [20], respectively. The rotational spectrum of indan has been reported a long time ago, measured with conventional MW spectroscopy [3]. Due to the very low value of the lc dipole moment component, it was not possible to measure the interstate lc-type transitions, which supply ⇑ Corresponding author. Fax: +39 051 2099456. E-mail address: [email protected] (W. Caminati). http://dx.doi.org/10.1016/j.jms.2015.08.001 0022-2852/Ó 2015 Elsevier Inc. All rights reserved.
directly the value of the vibrational tunneling splitting. This splitting was indirectly estimated by MW-RF double resonance experiments, observing the intensity change of some rotational transitions while changing the value of a radiofrequency signal applied to the Stark electrodes. Nowadays, it is feasible, with pulsed Fourier transform MW spectroscopy, to measure even quite weak transitions. For this reason, we decided to measure the lctype transitions of the most abundant isotopic species (normal) of indan, and in order to determine the molecular structure, the rotational spectra of its 13C-mono substituted isotopologues in natural abundance. The molecular shape of indan, inclusive of the principal axes and of the atom numbering used through the text, is shown in Fig. 1. 2. Experimental methods A commercial sample of indan 98% (Aldrich) was used without further purification. The spectra of the mono-substituted 13C isotopologues were analyzed in natural abundance. The rotational spectra have been measured with pulsed jet Fourier-transform microwave (FT-MW) spectroscopy [21], in a coaxially oriented beam-resonator arrangement-(COBRA)-type [22]. The spectrometer, working in the 6–18.5 GHz frequency region, is described elsewhere [23]. Helium, as carrier gas, was passed over indan at 60 °C temperature, at a backing pressure of about 0.2 MPa, and expanded through the pulsed valve (General valve, series 9, nozzle diameter 0.5 mm) into the Fabry–Perot cavity to about 1 103 Pa. The spectral line position was determined after Fourier transformation of the 8 k data point time domain signal, recorded at intervals of 100 ns. Each rotational transition is
46
L.B. Favero et al. / Journal of Molecular Spectroscopy 316 (2015) 45–48 Table 1 Spectroscopic parameters of the parent species of indan. Single values are common to both states v = 0 and v = 1. v=0
v=1 3531.0444(7)a
A (MHz) B (MHz) C (MHz) DJ (Hz) DJK (Hz) DK (kHz) d1 (Hz) d2 (kHz) DE01 (MHz) r/rexpb Nc
1498.3739(1) 1082.8714(1)
1498.3750(1) 1082.8724(1) 32.2(7) 52(4) 0.6(1) 7.1(4) 2.9(3) 22.364(1) 1.3 242
a
Fig. 1. Molecular shape, principal axes and atom numbering of indan.
Error in parentheses in units of the last digit. Root-mean-square deviation of the fit, referred to an estimated measure errors of 3 kHz and of 50 kHz for the transition frequencies of this investigation and for those of Ref. [3], respectively. c Number of lines in the fit (81 from the present work and 161 from Ref. [3]). b
split by Doppler effect, enhanced by the coaxial arrangement of the supersonic jet and resonator axes in the COBRA-FTMW spectrometer. The rest frequency is calculated as the arithmetic mean of the Doppler components. The estimated accuracy of frequency measurements is better than 3 kHz and lines separated by more than 7 kHz are resolvable. 3. Results and discussion 3.1. Rotational spectra The la-spectrum of the most abundant species has been easily identified, based on the rotational constants determined in the previous absorption room temperature MW investigation [3]. Although being intrastate transitions, they displayed small splittings, due to the difference of the rotational constants of the two tunneling states originated by the above mentioned ring puckering motion. To detect the much weaker lc-type lines we needed the maximum MW pulse power of the spectrometer and to warm up the sample to 60 °C before of the supersonic expansion, in order to increase its concentration in the beam. All these interstate transitions were split into two evenly separated (by ca 44.7 MHz, the double of the ring puckering splitting) component lines. We have been able to measure 67 la-type and 14 lc-type transitions, which have been fitted together with the rotational frequency reported in Ref. [3]. In the fit, the appropriate weight was given to the two sets of measurements. All transitions have been fitted simultaneously with a coupled Hamiltonian using the Pickett set of programs [24]. We used the following expressions:
H ¼ Ri HRi þ HCD þ DE01 ; with i ¼ 0; 1
or 13C8, 13C6 or 13C7), while the fifth isotopologue (13C2) is only 1%. A portion of the spectrum displaying the 312 211 transitions of the parent and 13C isotopologues in natural abundance is shown in Fig. 2. All rotational transitions are also reported in the Supplementary Material. They have been fitted in the same way as the normal species, supplying the spectroscopic constants of Table 2, where also the number of measured transitions is given. Due to the low number of experimental frequencies, the centrifugal distortion parameters have been fixed to the values of the normal species.
ð1Þ
where represents the rotational Hamiltonian for the state i. HCD accounts for the centrifugal distortion corrections, corresponding to the Ir-representation of Watson’s ‘‘S” reduced Hamiltonian [25], assumed to be the same for both states. DE01 is the energy difference between the v = 0 and v = 1 tunneling states. The spectroscopic constants obtained are reported in Table 1. A general improvement on the uncertainties of the experimental parameters has been achieved, with respect to Ref. [3]. In addition, the DE01 (=22.364(1) MHz) has been precisely determined from the fit, the rotational constants have been obtained separately for the two tunneling states, and all quartic centrifugal distortion constants have been fitted. We investigated then the rotational spectra of the five possible 13 C-monosubstituted isotopologues, in natural abundance. Four of these isotopologues have a concentration of ca. 2% of that of the most abundant species, because concern the substitution of one of two equivalent carbon atoms (13C1 or 13C3, 13C4 or 13C9, 13C5 HRi
Fig. 2. Portion of the spectrum with the 312 isotopologues in natural abundance.
211 transitions of the parent and 13C
Table 2 Experimental spectroscopic parameters of the 13C isotopologues (measured in natural abundance) of indan. Centrifugal distortion constants and the tunneling splitting have been fixed to the values of the parent species.
13
C1 C2 13 C4 13 C5 13 C6
(or
13
C3)
(or (or (or
13
C9) C8) C7)
13
a b c
13 13
A (MHz)
B (MHz)
C (MHz)
rb (kHz)
Nc
3494.71(1)a 3529.16(1) 3519.11(1) 3483.37(1) 3519.39(1)
1486.9640(3) 1472.0361(3) 1498.2566(3) 1493.8816(3) 1476.9880(3)
1073.5820(2) 1069.2427(2) 1081.7008(2) 1076.0163(2) 1070.5935(2)
0.9 0.6 0.6 0.8 0.9
9 9 9 9 9
Standard error in parentheses in units of the last digit. Standard deviation of the fit. Number of fitted transitions.
47
L.B. Favero et al. / Journal of Molecular Spectroscopy 316 (2015) 45–48 Table 3 Substitution coordinates (rs) of the carbon atoms in the principal axes system of parent indan. a (Å)
C2 C1(or C4(or C5(or C6(or a b
C3) C9) C8) C7)
b (Å)
c (Å)
rs
re
rs
re
rs
re
±2.4460(6)a ±1.6008(9) ±0.15(1) ±1.005(2) ±2.2123(7)
2.4366 1.6112 0.1925 1.0100 2.2169
0.07(2)ib ±1.223(1) ±0.696(2) ±1.407(1) ±0.697(2)
0.0 1.2204 0.7026 1.4128 0.7016
±0.288(5) ±0.14(1) ±0.06(3) ±0.01(13)ib ±0.05(3)
0.3136 0.1483 0.0634 0.0072 0.0578
Errors in parenthesis are expressed in units of the last digit. Imaginary value.
3.2. Structure and ab initio calculations
3.3. Barrier to ring puckering
From the rotational constants of the various isotopologues it has been possible to obtain the substitution cartesian coordinates of the carbon atoms by applying the Kraitchmann method [26] and using Costain’s uncertainties [27]. The results obtained results are shown in Table 3. We also calculated the MP2/6-311++G(d,p) [28] geometry of indan, which is reported in Table 4. The puckering angle s (180° – XYC2) is visualized in Fig. 1. X and Y represent the intermediate points between the C4–C9 and C1–C3 carbon atoms, respectively. In Table 4, the rs structural parameters of the C atoms are also shown. In turn, the principal axes ab initio cartesian coordinates of the C atoms are reported in Table 3 for comparison with the experimental values. The ab initio calculations also provided the rotational and first order centrifugal distortion constants, as well as the values of the la and lc components of the electric dipole moment (lb is zero by symmetry). All these parameters are shown in Table 5. They are in agreement with the experimental observations. The experimental rotational constants could be reproduced with the ab initio geometry within 1 or 2 MHz, after changing the value of s from 33.2 to 35°.
The ab initio calculations also provide the energy of the planar conformation of the main frame of indan, which difference with respect to the global minimum energy corresponds to the barrier to ring puckering. The value B2 = 733 cm1 has been deduced. This is quite similar to the value obtained by a MP2/6-31++G(d,p) calculation (B2 = 670 cm1) [19]. Both values are quite higher than the experimental values obtained by MW measurements (B2 = 434 cm1) [3] or by laser-induced fluorescence experiments (B2 = 488 cm1) [18]. As to the B2 value obtained with the previous MW analysis, we are satisfied with is because we confirmed the reliability of the indirectly obtained DE01 splitting (22.3 MHz, now precisely determined to be 22.364(1) MHz). Some experimental and theoretical values of the potential energy function are summarized in Table 6, while a graphical comparison of the MW and of the MP2/6-311++G(d,p) potential energy surfaces is given in Fig. 3.
Table 6 Experimental and theoretical values of the potential energy function of the ring puckering motion in indan.
Table 4 Theoretical (re, MP2/6-311++G(d,p)) geometry and rs coordinates of the C atoms of indan. Bond lengths (Å) re/rs C1–C2 C1–C9 C4–C5 C4–C9 C5–C6 C6–C7 C1–H10 C1–H11 C2–H12 C2–H13 C5–H16 C6–H17
re/rs
1.5440/1.547(7) 1.5126/1.543(9) 1.3978/1.359(9) 1.4051/1.392(3) 1.4024/1.402(5) 1.4032/1.394(3) 1.0947 1.0991 1.0959 1.0941 1.0881 1.0870
C1C2C3 C2C3C4 C3C4C9 C4C5C6 C5C6C7 C8C9C4 H10C1C9 H11C1C9 H12C2Y H13C2Y H16C5C4 H17C6C5
Significant dihedral angles (°)a s = 32.3/30(3) H10C1–C9C4 = 141.8 a
B2 (cm1)
s0 (°)
Valence angles (°) a
104.4/104.5(3) 102.4/103.2(8) 110.0/110.0(7) 119.0/118(1) 120.5/120.4(3) 120.5/121.5(8) 113.2 110.0 131.6 141.5 120.7 119.8
Experiments
Theory
MW
LIF (Ref. [18])
MP2/6-31++G(d,p) (Ref. [19])
MP2/6-311++G(d,p). This work
434 34.0
488 8.5a
670 30.8
733 32.3
Deduced from the value x1 = 0.14 Å at the energy minima.
H11C1–C9C4 = 97.6
All dihedral angles of the aromatic part very close to 0 or 180°.
Table 5 MP2/6-311++G** spectroscopic parameters of indan. A (MHz) B (MHz) C (MHz) la/D lc/D
3519 1496 1082 0.6 0.036
DJ (Hz) DJK (Hz) DK (Hz) d1 (Hz) d2 (Hz)
33.1 42.2 234 8.5 1.4 Fig. 3. MP2/6-311++G** and MW experimental shapes of the puckering B2 barrier.
48
L.B. Favero et al. / Journal of Molecular Spectroscopy 316 (2015) 45–48
4. Conclusions
References
With the present investigation we precisely measured the DE01 splitting, 22.364(1) MHz, related to the puckering of the metkylenic apex of the five membered ring of indan, and show that the previously indirectly measured splitting (DE01 = 22.3) was a reliable value. We also assigned the rotational spectra of the five possible 13C monosubstituted isotopologues. The obtained rs geometrical parameters are in agreement with the ab initio values. It is interesting to outline that the theoretical methods do not supply satisfactory values for the parameters concerning the puckering motion. The B2 barrier appear overestimated by about 60%, while the equilibrium value of the s angle is underestimated. Its r0 value (referred to the vibrational ground state) is determined, indeed, to be 35°, larger than the ab initio value, (re, 32.3°). Actually, having the vibrational ground state wavefunction a considerable density also in the region in between the two minima, we should have re > r0.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgments We acknowledge Italian MIUR (PRIN project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. W.L. thanks the China Scholarships Council (CSC) for a scholarship.
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). Supplementary data associated with this article can be found, in the online version, at http:// dx.doi.org/10.1016/j.jms.2015.08.001.
[11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
T.L. Smithson, J.A. Duckett, H. Wieser, J. Phys. Chem. 88 (1984) 1102. K.H. Hassan, J.M. Hollas, J. Mol. Spectrosc. 147 (1991) 100. W. Caminati, D. Damiani, G. Corbelli, L.B. Favero, Mol. Phys. 75 (1992) 857. W. Caminati, L.B. Favero, B. Velino, F. Zerbetto, Mol. Phys. 78 (1993) 1561. K.H. Hassan, J.M. Hollas, Chem. Phys. Lett. 169 (1990) 17. W. Caminati, D. Damiani, L.B. Favero, Mol. Phys. 79 (1993) 699. S. Sakurai, N. Meinander, J. Laane, J. Chem. Phys. 108 (1998) 3537. J.A. Duckett, T.C. Smithson, H. Wieser, Chem. Phys. Lett. 64 (1979) 261. K.H. Hassan, J.M. Hollas, Chem. Phys. Lett. 157 (1989) 183. W. Caminati, S. Melandri, G. Corbelli, B.L. Favero, R. Meyer, Mol. Phys. 80 (1993) 1297. S. Sakurai, N. Meinander, K. Morris, J. Laane, J. Am. Chem. Soc. 121 (1999) 5056. J. Laane, E. Bondoc, S. Sakurai, K. Morris, N. Meinander, J. Choo, J. Am. Chem. Soc 122 (2000) 2628. G. Pietraperzia, A. Zoppi, M. Becucci, E. Droghetti, E. Castellucci, Chem. Phys. Lett. 385 (2004) 304. Z. Kisiel, L. Pszczólkowski, G. Pietraperzia, M. Becucci, W. Caminati, R. Meyer, PCCP 6 (2004) 5469. P. Ottaviani, W. Caminati, Chem. Phys. Lett. 405 (2005) 68. J.A. Yang, K. Okuyama, K. Morris, J. Laane, J. Phys. Chem. A 109 (2005) 8290. E. Bondoc, T. Klots, J. Laane, J. Phys. Chem. A 104 (2000) 275. Z. Arp, N. Meinander, J. Choo, J. Laane, J. Chem. Phys. 116 (2002) 6648. A. Das, K.K. Mahato, S.S. Panja, T. Chakraborty, J. Chem. Phys. 119 (2003) 2523. Y. He, W. Kong, J. Chem. Phys. 122 (2005) 244302. T.J. Balle, W.H. Flygare, Rev. Sci. Instrum. 52 (1981) 33. J.-U. Grabow, W. Stahl, H. Dreizler, Rev. Sci. Instrum. 67 (1996) 4072. W. Caminati, A. Millemaggi, J.L. Alonso, A. Lesarri, J.C. Lopez, S. Mata, Chem. Phys. Lett. 392 (2004) 1. H.M. Pickett, J. Mol. Spectrosc. 148 (1991) 371. J.K.G. Watson, in: J.R. Durig (Ed.), Vibrational Spectra and Structure, Elsevier, New York/Amsterdam, 1977, pp. 1–89. 6. J. Kraitchman, Am. J. Phys. 21 (1953) 17. C.C. Costain, G.P. Srivastava, J. Chem. Phys. 35 (1961) 1903. Gaussian 09, Revision D.01, M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, O. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, Gaussian Inc, Wallingford CT, 2013.