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Rotational Spectra of Bicyclic Decanes: The Trans Conformation of (-)-Lupinine Michaela K. Jahn, David Dewald, Montserrat Vallejo-Lopez, Emilio Jose Cocinero, Alberto Lesarri, and Jens-Uwe Grabow J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp407671m • Publication Date (Web): 12 Sep 2013 Downloaded from http://pubs.acs.org on September 16, 2013

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The Journal of Physical Chemistry

Rotational Spectra of Bicyclic Decanes: The Trans Conformation of (-)-Lupinine

Michaela K. Jahn,† David Dewald,† Montserrat Vallejo-López,‡ Emilio J. Cocinero,§ Alberto Lesarri,*‡ Jens-Uwe Grabow*†

†

Institut für Physikalische Chemie & Elektrochemie, Lehrgebiet A, Gottfried-Wilhelm-

Leibniz Universität, Callinstraße 3A, D-30167 Hannover (Germany), ‡

Departamento de Química Física y Química Inorgánica, Facultad de Ciencias,

Universidad de Valladolid, E-47011 Valladolid (Spain), §

Departamento de Química Física, Facultad de Ciencia y Tecnología, Universidad del

País Vasco, Ap. 644, E-48080 Bilbao (Spain),

1 ACS Paragon Plus Environment


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ABSTRACT: The conformational and structural properties of the bicyclic quinolizidine alkaloid (-)-lupinine have been investigated in a supersonic jet expansion using microwave spectroscopy. The rotational spectrum is consistent with a single dominant trans conformation within a double-chair skeleton, which is stabilized by more than 10.4 kJ mol-1 with respect to other conformations. In the isolated conditions of the jet the hydroxy methyl side-chain of the molecule locks-in to form an intramolecular OH···N hydrogen bond to the electron lone-pair at the nitrogen atom. Accurate rotational constants, centrifugal distortion corrections and

14

N nuclear quadrupole coupling

parameters are reported and compared to ab initio (MP2) and DFT (M06-2X) calculations. The stability of lupinine is further compared computationally with epilupinine and decaline in order to gauge the influence of intramolecular hydrogen bonding, absent in these molecules.

Keywords:

Microwave

Spectroscopy,

Rotational

Spectra,

Conformational Analysis, Alkaloids


Supersonic

Jets,

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INTRODUCTION Decalin is a 10-carbon bicycle that was instrumental in proving the Sachse-Mohr theory of the chair conformation of 6-membered rings.1 The molecule exists as a double-chair made of two nearly undistorted cyclohexane units, adopting two alternative cis- (C2) or trans- (C2h) conformations (Scheme 1).2 Cis-decalin is a structural motif found in some steroids and may give rise by a double ring inversion to a pair of chair-chair enantiomeric species. Conversely, the most stable trans-decalin cannot double-invert and is achiral, so it serves as conformational anchor for a number of biologically relevant compounds (plausible twist configurations are expected to be considerably higher in energy3). A substitution of a single bridgehead carbon of decalin by a nitrogen atom produces the heterocyclic quinolizidine, which further originates several classes of alkaloids. We report here a rotational study of (-)-lupinine, one of the four steroisomers produced by introduction of a hydroxy methyl group next to the only carbon bridgehead in quinolizidine (Scheme 2). Lupinine is an alkaloid first extracted from seeds and herbs of the Lupinus luteus species, a plant of the Fabaceae family. It was later established by chemical methods that the natural product is the [(1R,9aR)Octahydro-1H-quinolizin-1-yl]methanol stereoisomer or (-)-lupinine,4,5 i.e., both the hydroxy methyl group at C-1 and the H atom at C-9 have absolute R configuration in a double-chair arrangement. This configuration was confirmed by X-ray diffraction studies of lupinine6 and its diastereoisomer epilupinine.7 An IR work further suggested that lupinine might be stabilized in solution by an O-H···N intramolecular hydrogen bond,8 which is only possible if the hydroxy methyl group is axial and cis to the nitrogen lone pair. However, other authors have argued that a small IR band associated to free OH would imply the simultaneous existence of rotamers.9 No evidence of



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intramolecular hydrogen bond was found in the crystal analysis, but there are no high resolution spectroscopic studies, except for the 13C NMR spectrum.10,11 In order to solve the conformational issues on (-)-lupinine we examined its rotational spectrum using Fourier-transform microwave (FT-MW) spectroscopy12,13,14 in a near-adiabatic supersonic expansion. This technique is capable of providing isotopicsensitive structural information and intramolecular dynamics for polar molecules and weakly-bound intermolecular clusters of moderate size (< ca. 30 heavy atoms).15 We address the conformational and structural properties of the free molecule, including the possibility of detecting higher-energy cis or trans-twist forms and the presence of intramolecular forces like hydrogen bonding, usually unnoticed in diffraction studies because of the strong intermolecular interactions in the crystal. Our study follows a previous interest on the structural properties of bicyclic and two-ring systems, which included rotational analysis of norbornanes (exo-2aminonorbornane16), tropanes (tropinone,17 scopine and scopoline18) and assemblies (anabasine,19 phenazone20). All these molecules exhibit apparently rigid rings, sometimes with strained geometries and unconventional structural parameters.21,22,23,2425 However, internal motions like inversion or internal rotation may affect their conformational equilibria and structural properties. The present study constitutes our first analysis of unsaturated six-membered fused rings and may sustain further attention on larger molecules and clusters based on this motif using rotational spectroscopy.


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EXPERIMENTAL AND COMPUTATIONAL METHODS The molecular properties of (-)-lupinine were probed in a supersonic jet expansion. The sample was commercially available (Maybridge, >98%) in minor quantities as a brown-orange solid and used without further purification. Lupinine has a relatively low melting point (70-80ºC) for its size, so it could be vaporized using a custom-made heating nozzle (φ=1.2 mm) attached to a commercial solenoid-driven (Parker Hannifin/General Valve) pulsed valve. A supersonic jet was created seeding a flow of pure argon at stagnation pressures of ca. 3 bar. Nozzle temperatures of 90ºC were satisfactory to observe the rotational transitions. The microwave spectrum was observed with a Balle-Flygare-type Fouriertransform microwave spectrometer at the University of Hannover, using a coaxially oriented beam and resonator axes (COBRA26) arrangement. This instrument has been described extensively,27 so only brief experimental details are given here. The BalleFlygare design uses impulse-excitation techniques12,13 and a narrow bandwidth (ca. 1 MHz) microwave resonator14 for interaction between the probing radiation and the supersonic jet. The use of the resonator reduces the power requirements (here 1 µs at 16 dBm) and considerably amplifies the molecular transient emission. The freeinduced-decay (FID) at microwave frequencies is recorded in the time-domain and down-converted to the radio-frequency region, where it is digitized and Fouriertransformed to produce a frequency-domain spectrum. The transition frequencies appear split by the Doppler effect because of the coaxial arrangement of the jet and the resonator axis, so the rest frequencies are taken as the average of the two frequency components. Frequency measurements have an estimated accuracy better than 0.5 kHz (unblended signals at good signal-to-noise ratios). Transitions separated by more than 6 kHz are resolvable.



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The computational methods proceeded in two steps. Initially, molecular mechanics (Merck molecular force field28) was used for screening the hundreds of structures generated in a conformational search using Macromodel.29 In this process we used both Monte-Carlo and “Large-scale low-modes” conformational search algorithms. A set of 57 structures with relative energies below 50 kJ mol-1 were later fully reoptimized using DFT and ab initio methods. Following previous experiments we compared the MP2 perturbation method and the hybrid meta-GGA (Generalized Gradient Approximation) M06-2X functional,30 empirically accounting for dispersion interactions.31 The combination of these methods with the Pople triple-ζ basis set (6311++G(d,p)) usually represents a satisfactory balance in terms of spectroscopic accuracy and computational cost. The molecular properties predicted theoretically included conformational energies, structural and rotational parameters, electric properties (nuclear quadrupole coupling tensor and electric dipole moments) and vibrational frequencies. All conformations were checked for positive vibrational frequencies. The quartic centrifugal distortion constants were calculated from the harmonic force field. All ab initio and DFT computations used Gaussian09.32


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RESULTS Conformational search. As expected all low-energy structures of (-)-lupinine are predicted in a double-chair configuration (both MP2 and M06-2X). Among the chair-chair structures both cis and trans quinolizidine skeletons were detected, but the trans form is much more stable and the first cis form is observed only at conformational thermal energies above ∆G=17.3-21.7 kJ mol-1 (electronic energies 18.8-23.1 kJ mol-1). For this reason, most of the lowest-lying conformations are generated by the two internal rotation axes of the hydroxy methyl group (bonds C1-C10 and C10-O in Scheme 2) in a trans configuration. The relative energies and molecular properties of the eight most stable conformations of lupinine are collected in Table 1. Despite the experimental conditions of a supersonic jet are not at thermodynamic equilibrium we report in table 1 both electronic and Gibbs free energies at 298K, which have been instrumental to interpret jet-spectroscopy population ratios when combined with a description of the potential energy surface and conformational relaxation pathways.33,34 Conformers are classified as cis/trans double-chairs and additionally identified according to the dihedrals of the methoxy group τ1 (H-O-C10-C1) and τ2 (O-C10-C1C9) with conventional identifiers cis (C), gauche (G+, G-), anticlinal (A+, A-) and trans (T), as in Figure 1. Trans skeletons sharing a boat and twist configurations, expected at relatively higher energies, were first encountered around ∆E=20.2-21.0 kJ mol-1 (∆G=20.4-21.2 kJ mol-1), i.e. slightly below the first cis form. Rotational Spectrum. The search of the microwave spectrum started in the cmwave region around 15 GHz, and soon revealed a set of rotational transitions with the characteristic splittings expected from the nuclear quadrupole coupling hyperfine interaction arising from a single 14N amine nucleus.35 A section of the spectrum and an illustrative transition are shown in Figure 2. Typically this hyperfine interaction splits 7 ACS Paragon Plus Environment


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each rotational transition into the three most intense components separated by less than 200 kHz, thus easily resolved with the FT-MW technique. The rotational transitions were eventually assigned using trial rotational constants obtained from the theoretical predictions. After a sequence of iterative fits all observed transitions were ascribed to a single near-prolate (κ ∼ -0.62) asymmetric rotor with the three µa-, µb- and µc-selection rules active. The Watson’s semirigid-rotor Hamiltonian HR (A-asymmetric reduction, Ir representation)36 supplemented with a nuclear quadrupole term HQ accurately reproduced all frequency measurements within the experimental accuracy. The experimental dataset comprised angular momentum quantum numbers in the range J=512. All five quartic centrifugal distortion constants could be determined, though ∆K was found poorly sensitive to the present dataset. The hyperfine observations were reproduced taking into account only the diagonal elements of the nuclear quadrupole coupling tensor (χαα, α=a, b, c), a common behavior for amine groups. The coupling tensor linearly relates to the electric field gradient (q) at the quadrupolar nucleus according to χ=eQq (Q quadrupolar moment). The resulting rotational parameters of ()-lupinine are shown in Table 2, while the full set of measured transitions is collected in the Supporting information (Table S1). In this work, the small amount of sample available and the spectrum intensity prevented the observation of other isotopologues in natural abundance (0.4-1.1%). Scans targeting other plausible conformations did not provide evidence for any other species of (-)-lupinine in the spectrum. The rotational parameters were used to determine which of the (-)-lupinine conformations is the carrier of the spectrum through comparison of the rotational constants and the nuclear quadrupole coupling parameters with the quantum-chemically predicted values. The experimental rotational constants show a good concordance with the theoretical structure at the global minimum, with relative differences below 0.8%. 8 ACS Paragon Plus Environment

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The agreement is much worse for all other conformations in Table 1 (6-20%), essentially excluding any of them. This conclusion is reinforced through the experimental nuclear quadrupole coupling constants. In this molecule the differences in the electronic environment around the nitrogen atom are expected to be small, as nitrogen is locked in a similar structural skeleton in all the predicted conformations. Additionally, the orientation of the nitrogen group in the principal inertial axes system does not change significantly, so the nuclear quadrupole coupling constants do not exhibit large changes between the different conformations. Nevertheless, the experimental observations still match preferentially the predictions for the global minimum. The comparison of the experimental centrifugal distortion constants with the theoretical predictions, despite being less significant and limited to the harmonic approximation, also offers a reasonable agreement for the most stable conformation.



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DISCUSSION We examined the conformational landscape of the bicyclic quinolizidine alkaloid of (-)-lupinine in a supersonic jet expansion using rotational spectroscopy. The microwave spectrum detected a single dominant conformation, for which accurate rotational constants and 14N nuclear quadrupole coupling parameters were determined. The quinolizidine skeleton preference for the trans double-chair arrangement was confirmed in the conformational search and evaluated quantitatively using ab initio calculations. Double-chairs with a cis configuration are predicted only for electronic energies above 18.8-23.1 kJ mol-1 (Tables 1 and S2, Gibbs free energy: 17.3-21.7 kJ mol-1). The distribution of low-energy conformations is thus based on the most stable hydroxy methyl orientations. The observed conformation of lupinine characteristically exhibits a stabilizing O-H···N intramolecular hydrogen bond between the diaxial hydroxy group and the non-bonding electron lone-pair at the nitrogen atom, which would not be possible in epilupinine because of its equatorial hydroxy methyl group. The predicted hydrogen bond distance (MP2: r(H···N) = 1.92 Å) classifies as a moderate hydrogen bond37 and can be compared with related values both in the gas-phase and crystal structures. In the gas-phase the comparisons are especially useful with intermolecular contacts in weakly-bound dimers, where steric constrains are reduced. A good agreement is found with the distances observed by rotational spectroscopy in monohydrated clusters of amines, where the water molecule acts as proton donor to the nitrogen atom. Examples include the five and six-membered rings of pyrrolidine38 and phenylethylamine,39 with effective r(H···N) distances of 1.89(1) Å and 1.955(6) Å, respectively. For other hydrated clusters only the r(N···O) distances are reported (piperidine:40 2.916 Å, morpholine:41 2.911 Å, aniline:42 3.03 Å) so comparisons are not entirely univocal. A similar interaction is observed for ammonia clusters linked to a


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hydroxy group, as in methyl lactate···NH3 (r(O-H···N)=1.928 Å) and glycidol···NH3 (r(O-H···N)=1.916-1.917 Å), practically with the same hydrogen bond distance.43,44 Larger contacts are observed however for aminoethanol···NH3 and tert-butanol···NH3 (2.009 Å and 2.03 Å, respectively).45,46 The results of a survey of more than 400 crystal structures with a O-H···N(sp2) interaction offered consistent distances in the range r(O···N)=2.81-3.08 Å.47 The degree of non-linearity observed in lupinine (∠(OH···N)=148.5º) is well within the crystal survey data (∠(O-H···N) > 140º) despite the constrains imposed by the ring. In consequence, the presence of the O-H···N interaction in lupinine is confirmed. The presence of a free hydroxy group in solution, quantified as ca. 11% using low-resolution IR spectroscopy,9 cannot be totally ruled out from the gasphase analysis, but does not correspond with the conformational preferences of the isolated molecule. We estimated the stabilizing effect of the intramolecular hydrogen bond by comparing the conformational free energies of the cis/trans forms of lupinine, epilupinine and decalin with computational methods. The compilation in Table S3 (MP2) shows a striking difference of ca. 10 kJ mol-1 between the case of lupinine (∆G=21.7 kJ mol-1) and those of decalin (11.8 kJ mol-1) and epilupinine (11.6 kJ mol-1), where intramolecular hydrogen bonding is not possible. Moreover, the cis forms of decalin and epilupinine have very similar relative energies, pointing to a contribution of the intramolecular hydrogen bond to the stability of epilupinine of ca. 10.1 kJ mol-1. The existence of free OH groups in solution, if any, should eventually account for this energy penalty. All six cis/trans structures are additionally tabulated in the Supporting Information (Tables S4-S6) We should note that the intramolecular hydrogen bond of lupinine was not observed in the X-ray diffraction study,6 where the hydroxy group is oriented out of the ring to establish an intermolecular hydrogen bond with other molecules in the crystal. 11 ACS Paragon Plus Environment


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This situation is common to other diffraction studies, not always revealing the most stable conformation for the isolated molecule.17,19 As a consequence, the importance of gas-phase studies to define the intrinsic conformational landscapes of isolated molecules is again recognized. The first trans-twist conformation is predicted at ∆G=20.4-21.2 kJ mol-1 (MP2) above the global minimum of lupinine, so similarly to the cis form it is not populated at the temperatures of the jet and other experiments would be needed to examine its detailed structure in molecules where this conformation is thermally accessible. We observed a good agreement between the experimental and theoretical data in Tables 1 and 2. As in previous experiments we were interested to check the predictive capabilities of the new functionals empirically accounting for dispersion interactions, in particular Truhlar’s M06-2X. For lupinine, with a relatively rigid heavy-atom skeleton, the consistence between the MP2 and M06-2X methods is very good. This consistency extends to the prediction of the nuclear quadrupole coupling constants, where usually MP2 behaves better.16 This evidence accumulates to other studies suggesting that the combination of M06-2X and a triple-ζ basis set represents a satisfactory cost-to-benefit alternative for spectroscopic predictions of organic molecules and weakly-bound clusters,48 considerably improving the performance of the B3LYP method. We emphasize the importance of jet-cooled rotational studies for structural purposes and benchmarking of theoretical models. The results of this work are valuable in the analysis of larger molecules based on the bicyclic decane motif, in particular steroids, where cis and trans forms may exist and for modelling intermolecular complexes involving the title compound, in particular hydration clusters. Other related bicyclic compounds are now being studied rotationally.


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ASSOCIATED CONTENT Supporting Information. Tables S1-S6 with the observed rotational transitions, conformational search, and ab initio structures of lupinine, epilupinine and decalin. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding authors. A.L. Phone: +34(983)185895. Fax: +34(983)423013. Email: [email protected]. Website: www.uva.es/lesarri J.-U.G.

Phone:

+49(511)7623163.

Fax:

+49(511)7624009.

Email:

jens-

[email protected]. Website: www.grabow.pci.uni-hannover.de

ACKNOWLEDGEMENT We thank the Deutsche Forschungsgemeinschaft and the Land Niedersachen for financial support. M.V.L., A.L. and E.J.C. acknowledge funding from the Spanish MICINN and MINECO (CTQ2011-22923, CTQ2012-39132-C02-02), the Basque Government (Consolidated Groups, IT520-10) and the UPV/EHU (UFI11/23). E.J.C. and M.V.L. thank also the MICINN for a “Ramón y Cajal” contract and a FPI grant, respectively.



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REFERENCES (1) Eliel, E. L.; Wilen, S. H. Stereochemistry of Organic Compounds, Wiley: New York, 1994. (2) Van den Enden, L.; Geise, H. J., “The Molecular Structure of Cis- and Trans-Bicyclo [4.4.0] Decane in the Gas Phase, Studied by Electron Diffraction and Molecular Mechanics”, J. Mol. Struct, 1978, 44, 177-185. (3) Durig, J. R.; Zheng, C.; El Defrawy, A. M.; Ward, R. M.; Gounev, T. K.; Ravindranath, K. N.; Rao, R., “On the Relative Intensities of the Raman Active Fundamentals, r0 Structural Parameters, and Pathway of Chair–boat Interconversion of Cyclohexane and Cyclohexane-D12” J. Raman Spectrosc. 2009, 40, 197-204. (4) Leonard, N. J. “Lupin Alkaloids”, in The Alkaloids: Chemistry and Physiology, Vol. III (Ed.: Manske, R. H. F.; Holmes, H. L.) chap. 19, Academic Press: New York, 1953; and Vol. VII (Ed.: Manske, R. H. F.), chap. 14, Academic Press: New York, 1960. (5) Bohlmann, F.; Schumann, D. “Lupine Alkaloids”, in The Alkaloids: Chemistry and Physiology, Vol. IX (Ed.: Manske, R. H. F.) chap. 5, Academic Press: New York, 1967. (6) Koziol, A.; Kosturkiewicz, Z, Podkowinska, H., “Structure of the Alkaloid Lupinine”, Acta Cryst. 1978, B34, 3491-3494. (7) Koziol, A.; Kosturkiewicz, Z, Podkowinska, H., “Structure of (+)-Epilupinine”, Acta Cryst. 1980, B36, 982-983. (8) Thomas, A. F.; Vipond, H. J.; Marion, L., “The Papilionaceous Alkaloids: XXI. The Alakaloids of Lupinus Pilosus Walt. and the Structure of Tetralupine”, Can. J. Chem. 1955, 33, 1290-1294. (9) Aaron, H. S.; Ferguson, C. P., “Conformational Analysis of Intramolecular Bonded Amino Alcohols: The Conformational Free Energies of Some Intramolecular OH ⋯ N Hydrogen Bonds”, Tetrahedron, 1974, 30, 803-811. (10) Podkowinska, H.; Skolik, J., “13C NMR Spectroscopic Study of Lupinine and Epilupinine Salts and Amine Oxides”, Org. Mag. Res. 1984, 22, 379-384. (11) Bohlmann, F.; Zeisberg, R. “Lupine Alkaloids. XLI. Carbon-13 NMR Spectra of Lupine-Alkaloids”, Chem. Ber., 1975, 108, 1043-1051. (12) Grabow, J.-U.; Caminati, W., “Microwave Spectroscopy – Experimental Techniques”, in Frontiers of Molecular Spectroscopy (Ed.: J. Laane), chap. 14, pp. 383-454, Elsevier: Amsterdam, 2008. (13) Schmalz, T. G.; Flygare, W. H. Coherent “Transient Microwave Spectroscopy and Fourier Transform Methods”, in Laser and Coherence Spectroscopy (Ed.: Steinfeld, J. I.), chap. 2, pp. 125-196, Plenun Press: New York, 1978. (14) Balle, T. J.; Flygare, W. H., “Fabry–Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source” Rev. Sci. Instrum. 1981, 52, 33-45. (15) Caminati, W.; Grabow, J.-U. , “Microwave Spectroscopy – Molecular Systems”, in Frontiers of Molecular Spectroscopy (Ed.: J. Laane), chap. 15, pp. 455-552, Elsevier: Amsterdam, 2008. (16) Cocinero, E. J.; Lesarri, A.; Écija, P.; Millán, J.; Fernández, J. A.; Castaño, F., “Discriminating the Structure of Exo-2-Aminonorbornane Using Nuclear Quadrupole Coupling Interactions” J. Chem. Phys. 2011, 134, 164311(1-8). (17) Cocinero, E. J.; Lesarri, A.; Écija, P.; Grabow, J.-U.; Fernández, J. A.; Castaño, F., “N-Methyl Stereochemistry in Tropinone: The Conformational Flexibility of the Tropane Motif”, Phys. Chem. Chem. Phys. 2010, 12, 6076-6083. (18) Écija, P.; Cocinero, E. J.; Lesarri, A.; Basterretxea, F. J.; Fernández, J. A.; Castaño, F., “The Distorted Tropane of Scopoline”, Chem. Phys. Chem. 2013, 14, 1830-1835. (19) A. Lesarri, E. J. Cocinero, L. Evangelisti, R. D. Suenram, W. Caminati, J.-U. Grabow, “The Conformational Landscape of Nicotinoids: Solving the Conformational Disparity of Anabasine”, Chem. Eur. J. 2010, 16, 10214-10219. (20) Écija, P.; Cocinero, E. J.; Lesarri, A.; Fernández, J. A.; Caminati, W.; Castaño, F., “Rotational Spectroscopy of Antipyretics: Conformation, Structure, and Internal Dynamics of Phenazone”, J. Chem. Phys., 2013, 138, 114304(1-7). (21) Levin, M. D.; Kaszynski, P.; Michl, J., “Bicyclo[1.1.1]pentanes, [n]Staffanes, [1.1.1]Propellanes, and Tricyclo[2.1.0.0(2,5)]pentanes”, Chem. Rev. 2000, 100, 169-234. (22) Kisiel, Z.; Desyatnyk, O.; Białkowska-Jaworska, E.; Pszczołkowski, L., “The Structure and Electric Dipole Moment of Camphor Determined by Rotational Spectroscopy”, Phys. Chem. Chem. Phys. 2003, 5, 820-826. (23) Choplin, A., “Microwave Spectrum and Dipole Moment of Norbornane”, Chem. Phys. Lett. 1980, 71, 503-506.


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CAPTIONS FOR FIGURES

Scheme 1. Cis- and trans- conformations of bicyclic decanes. Scheme 2. The diastereoisomers of lupinine and epilupinine. Figure 1. Predicted most stable conformations of (-)-lupinine. Figure 2. A section of the microwave spectrum of (-)-lupinine and a rotational transition illustrating the 14N (I=1) nuclear quadrupole coupling hyperfine effects in the molecule (hyperfine components labeled with quantum number F, with F=I+J).


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Table 1. Ab initio (MP2) and DFT (M06-2X) predictions for the most stable conformations of (-)-lupinine.

Conf 1 a Conf 2 Conf 3 Conf 4 Conf 5 Conf 6 Conf 7 Conf 8 Trans-CGTrans-TT Trans-TG+ Trans-G+T Trans-G-T Trans-G+G+ Twist-CGCis-G+G+ A (MHz)b 1425.8/1422.5 1503.3/1504.9 1213.0/1212.9 1485.9/1490.0 1485.1/1492.4 1208.7/1209.1 1388.1/1383.4 1246.6/1245.1 B (MHz) 815.1/815.7 719.2/720.6 869.6/871.7 720.3/721.8 718.1/719.6 869.7/872.8 878.9/880.0 827.6/834.7 C (MHz) 677.1/672.2 559.9/560.6 583.0/583.0 559.6/561.2 556.6/560.0 585.0/584.3 721.0/718.5 586.7/586.0 ∆J (kHz) 0.023/0.024 0.018/0.018 0.028/0.026 0.018/0.017 0.019/0.019 0.027/0.026 0.028/0.029 0.045/0.047 ∆JK (kHz) 0.065/0.068 0.071/0.077 0.045/0.040 0.066/0.059 0.082/0.086 0.043/0.045 0.051/0.052 -0.10/-0.11 ∆K (kHz) 0.0022/0.0014 0.094/0.085 -0.014/-0.010 0.10/0.093 0.094/0.079 -0.011/-0.013 0.0059/0.012 0.13/0.13 δJ (kHz) 0.0027/0.0028 0.0001/-0.0002 0.0063/0.0058 0.0002/0.0002 -0.0003/-0.0006 0.0057/0.0050 0.0030/0.0032 0.017/0.017 δK (kHz) 0.020/0.020 0.074/0.077 0.062/0.057 0.071/0.067 0.081/0.083 0.059/0.061 0.023/0.021 0.034/0.033 χaa (MHz) 2.0/2.2 2.1/2.2 2.1/2.3 2.0/2.2 2.0/2.2 2.1/2.3 2.3/2.5 1.9/2.2 χbb (MHz) 1.1/1.1 1.7/1.8 1.6/1.7 1.7/1.8 1.7/1.8 1.5/1.6 0.2/0.2 1.7/1.8 χcc (MHz) -3.1/-3.4 -3.8/-4.0 -3.7/-4.0 -3.7/-4.0 -3.7/-4.0 -3.6/-4.0 -2.5/-2.8 -3.6/-3.9 |µa| (D) 1.3/1.4 1.1/1.1 0.26/0.21 1.4/1.4 1.3/1.2 0.66/0.68 0.96/1.0 0.33/0.31 |µb| (D) 1.1/1.1 0.50/0.49 0.85/0.81 1.7/1.6 0.46/0.46 1.2/1.1 1.4/1.4 1.2/1.2 |µc| (D) 2.4/2.3 0.65/0.64 0.83/0.83 0.44/0.46 1.6/1.7 1.4/1.5 2.2/2.2 0.20/0.08 |µTOT| (D) 2.9/2.9 1.4/1.3 1.2/1.2 2.2/2.2 2.1/2.1 2.0/2.0 2.8/2.8 1.3/1.3 c 27.8 -176.7 -179.7 74.4 -64.9 63.4 34.4 61.3 τ1 -52.8 -171.8 89.1 -173.6 -173.6 85.1 -60.7 51.9 τ2 r(N···H-O) (Å) 1.92 1.92 ∆E (kJ mol-1)d 0.0/0.0 14.8/14.6 16.5/15.7 17.0/16.0 17.7/16.1 18.2/16.6 20.4/20.2 22.0/25.0 ∆EZPE (kJmol-1) 0.0/0.0 17.0/12.1 13.5/12.5 14.7/14.1 15.1/13.5 15.9/13.3 21.0/20.2 23.1/18.8 ∆G (kJ mol-1) 0.0/0.0 10.4/10.4 11.5/11.1 12.2/13.0 13.3/11.6 13.6/11.8 21.2/20.4 21.7/17.3 a

MP2 and M06-2X values, respectively, for each rotational parameter (basis set 6-311++G(d,p)). bRotational constants (A, B, C); Watson’s A-reduction quartic centrifugal distortion constants (∆J, ∆JK, ∆K, δJ, δK); nuclear quadrupole coupling tensor elements (χαβ, α, β =a, b, c) and electric dipole moment components (µα, α=a, b, c. 1 D≈3.336×10-30 Cm) referred to the principal inertial axes. cTorsion dihedrals of the ethoxy group τ1(H-O-C10-C1), τ2(O-C10-C1-C9) and hydrogen bond distance according to MP2 calculations. d Relative energies respect to the global minimum (zero-point-energy corrected) and Gibbs free energies at 298.15 K and 1 atm.

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Table 2. Rotational parameters of (-)-lupinine (conformer trans-CG-).

A (MHz)a B (MHz) C (MHz) ∆J (kHz) ∆JK (kHz) ∆K (kHz) δJ (kHz) δK (kHz) χaa (MHz) χbb (MHz) χcc (MHz) Nb σ (kHz)

1414.12625(12)c 811.67170(13) 671.53001(15) 0.02550(52) 0.0639(16) 0.0037(22) 0.00313(17) 0.0304(34) 1.9973(93) 1.062(13) -3.059(22) 92 1.9

a

Parameter definition as in Table 1. bNumber of fitted transitions and standard deviation of the fit. cStandard error in parentheses in units of the last digit.


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Cis- and trans- conformations of bicyclic decanes. 170x273mm (300 x 300 DPI)



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The diastereoisomers of lupinine and epilupinine. 116x144mm (300 x 300 DPI)


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Predicted most stable conformations of (-)-lupinine. 260x662mm (300 x 300 DPI)



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A section of the microwave spectrum of (-)-lupinine and a rotational transition illustrating the 14N (I=1) nuclear quadrupole coupling hyperfine effects in the molecule (hyperfine components labeled with quantum number F, with F=I+J). 96x80mm (300 x 300 DPI)


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30x18mm (300 x 300 DPI)


Rotational Spectra of Bicyclic Decanes - American Chemical Society

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