Puckering Motion Research Articles

Impact of the Dynamic Electron Correlation on the Unusually Long Excited-State Lifetime of Thymine.

Non-radiative relaxation of the photoexcited thymine in the gas phase shows an unusually long excited-state lifetime, and, over the years, a number of models, i.e., S1-trapping, S2-trapping, and S1&S2-trapping, have been put forward to explain its mechanism. Here, we investigate this mechanism using non-adiabatic molecular dynamics (NAMD) simulations in connection with the recently developed mixed-reference spin-flip time-dependent density functional theory (MRSF-TDDFT) method. We show that the previously predicted S2-trapping model was due to an artifact caused by an insufficient account of the dynamic electron correlation. The current work supports the S1-trapping mechanism with two lifetimes, τ1 = 30 ± 1 fs and τ2 = 6.1 ± 0.035 ps, quantitatively consistent with the recent time-resolved experiments. Upon excitation to the S2 (ππ*) state, thymine undergoes an ultrafast (ca. 30 fs) S2→S1 internal conversion and resides around the minimum on the S1 (nOπ*) surface, slowly decaying to the ground state (ca. 6.1 ps). While the S2→S1 internal conversion is mediated by fast bond length alternation distortion, the subsequent S1→S0 occurs through several conical intersections, involving a slow puckering motion.

Understanding Ring Puckering in Small Molecules and Cyclic Peptides.

The geometry of a molecule plays a significant role in determining its physical and chemical properties. Despite its importance, there are relatively few studies on ring puckering and conformations, often focused on small cycloalkanes, 5- and 6-membered carbohydrate rings, and specific macrocycle families. We lack a general understanding of the puckering preferences of medium-sized rings and macrocycles. To address this, we provide an extensive conformational analysis of a diverse set of rings. We used Cremer–Pople puckering coordinates to study the trends of the ring conformation across a set of 140 000 diverse small molecules, including small rings, macrocycles, and cyclic peptides. By standardizing using key atoms, we show that the ring conformations can be classified into relatively few conformational clusters, based on their canonical forms. The number of such canonical clusters increases slowly with ring size. Ring puckering motions, especially pseudo-rotations, are generally restricted and differ between clusters. More importantly, we propose models to map puckering preferences to torsion space, which allows us to understand the inter-related changes in torsion angles during pseudo-rotation and other puckering motions. Beyond ring puckers, our models also explain the change in substituent orientation upon puckering. We also present a novel knowledge-based sampling method using the puckering preferences and coupled substituent motion to generate ring conformations efficiently. In summary, this work provides an improved understanding of general ring puckering preferences, which will in turn accelerate the identification of low-energy ring conformations for applications from polymeric materials to drug binding.

Position of the Benzene Ring Substituent Regulates the Excited-State Deactivation Process of the Benzyluracil Systems.

A systematic theoretical study of the regulating effect of the substituent position on the photoinduced deactivation process of the benzyluracil systems has been performed based on the high-level static electronic structure calculations and on-the-fly full-dimensional excited-state dynamics simulations. Similarities and differences coexist for the two systems by comparative studies on the photoinduced deactivation process of the 5-benzyluracil (5-BU) and 6-benzyluracil (6-BU) systems. They both obey an S2 → S1 → S0 two-step decay pattern, and the decay coordinates of the S2 → S1 and S1 → S0 processes are mainly driven by the elongation of the bridging bond and the out-of-plane ring deformation motion, respectively. However, the puckering motion occurring at the C2 atom in the uracil fragment dominates the decay pathway of the 5-BU system. On the contrary, the puckering motion at the C5 atom in the benzene fragment mainly drives the decay coordinate of the 6-BU system. Therefore, the substituent position could play significant roles in the deactivation process of the benzyluracil systems. Moreover, the S1 → S0 decay process of the 6-BU system consists of five pathways, possessing a more complex deactivation picture than the 5-BU system. The fitted time scale of the puckering motion is compatible with the experimentally observed lifetimes. This work provides a fundamental understanding of the photophysical and photochemical properties of the benzyluracil systems and can give rational suggestions to further design or regulate the bionic molecular systems.

Theoretical Calculations, Microwave Spectroscopy, and Ring-Puckering Vibrations of 1,1-Dihalosilacyclopent-2-enes

High level theoretical CCSD/cc-pVTZ computations have been carried out to calculate the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2SiCPF2) and 1,1-dichlorosilacyclopent-2-ene (2SiCPCl2). The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2SiCPBr2) was obtained by ab initio MP2/cc-pVTZ computations. The parent silacyclopent-2-ene (2SiCP) is puckered with a 49 cm-1 barrier to planarity, 2SiCPF2 has a planar ring system, 2SiCPCl2 has a calculated tiny 4 cm-1 barrier but is essentially planar, and the dibromide has a calculated barrier of 36 cm-1. Microwave spectra of seven isotopic species of 2SiCPF2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The a-type and b-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent, 29Si, 30Si, and all singly-substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar. The experimentally determined rotational constants and geometrical parameters agree very well with the theoretical values and confirm the planarity of the five-membered ring. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid.

New insight into the ultrafast excited state deactivation mechanism of guanosine in the gas phase

The deactivation process of guanosine molecule on the first singlet excited state in the gas phase has been explored through the density functional theory (DFT/TDDFT) and the complete active space self-consistent field (CASSCF) methods. To unravel the decay mechanism of this system, the topography of excited potential energy surfaces relevant to the deactivation process were mapped. Two deactivation decay channels characterized by the excited state intramolecular proton transfer process and the puckering motion of ring were explored, and the latter one was proposed to play significant roles in the deactivation process of guanosine for the barrierless energy profile. However, the probability for the excited state intramolecular proton transfer process pathway is lower because of the existence of an energy barrier. This conclusion was further confirmed by the two minimum energy conical intersections (MECIs) located between S1 and S0 states, characterized by the ESIPT process and pyramidalization of the C2 atom, respectively. In addition, by comparing the decay mechanism with the previously reported adenosine and pyrimidine nucleosides, it was found that two competitive decay pathways coexist in the purine nucleosides, which is different from the pyrimidine nucleosides. As the next step in the bottom-up investigation of the DNA photostability properties, the new mechanism of the guanosine in gas phase will be conducive for more comprehensive understanding of the photostability of DNA and RNA molecules and provide theoretical guidance for the investigation of the different decay mechanism of the purine nucleosides and pyrimidine nucleosides.

Analysis of the Coriolis perturbed rovibrational spectrum of the C[sbnd]O asymmetric stretch and C[sbnd]C symmetric stretch of trimethylene oxide

Rotationally-resolved vibrational spectra of trimethylene oxide (c-C3H6O) were recorded using synchrotron radiation from the Canadian Light Source between 400 and 1200 cm−1 with a resolution of 0.000959 cm−1. The dense spectra are composed of transitions arising from excitation of several fundamental vibrations as well as hotbands and combination bands involving the very low frequency ring puckering (ν18) vibration at ~53 cm−1. To date, 3452 transitions have been assigned corresponding to the CO asymmetric stretch (ν23) at ~1008 cm−1 and the CC symmetric stretch (ν6) at ~1033 cm−1 which are coupled via second order c-type Coriolis interaction. An additional perturbation from a lower energy state was observed and attributed to a first order b-type interaction with a state that is likely the combination of the CC asymmetric stretching (ν24) at ~937 cm−1 and ring puckering motions. An additional 916 transitions involving the CO stretching mode were assigned to two hotbands that originate in the first and second excited ring puckering states of trimethylene oxide. Accurate band centers for the ν6, ν23, ν23 + ν18, ν23 + 2ν18 and ν24 + ν18 vibrational states were determined and are compared with harmonic and anharmonic frequency estimates at the MP2 and DFT B3LYP levels using the 6-311++G(d,p) basis set. The analysis of additional features at lower frequency is in progress and will be reported in a subsequent article.

Rotational spectroscopy of chiral tetrahydro-2-furoic acid: Conformational landscape, conversion, and abundances.

The conformational landscape of tetrahydro-2-furoic acid (THFA), a chiral carboxylic acid which is often used as a precursor in syntheses of pharmaceuticals, was investigated using rotational spectroscopy and theoretical modeling. Extensive manual searches were carried out to identify possible conformers related to the relative orientations of the carbonyl and hydroxyl groups in the COOH functional group, the COOH rotation, and the ring puckering motions in the system. A large number of initial conformational geometries were generated in parallel using a joint semiempirical-molecular dynamics simulation program. The final geometry optimizations were carried out at the B3LYP-D3(BJ)/def2-TZVP, B3LYP-D3(BJ)/6-311++G(2d,p), and MP2/6-311G++(2d,p) levels of theory. Eight conformers within a relative energy span of 10 kJ mol-1 after zero-point energy corrections were identified. Rotational spectra of three conformers were detected experimentally and assigned, as were the spectra of all the 13C isotopologues of the most stable conformer. Based on the achieved experimental sensitivity and the predicted relative abundances at the sample source, some conformers are unexpectedly missing or experience significant depletion, whereas others show noticeable enrichment. Detailed analyses of the conformational conversion barriers were carried out to satisfactorily explain the observed phenomena. The combined experimental rotational spectroscopic and theoretical investigation provides significant insights into the complex conformational landscape of THFA.

UV photofragmentation and IR spectroscopy of cold, G-type β-O-4 and β-β dilignol-alkali metal complexes: structure and linkage-dependent photofragmentation.

Ultraviolet photofragmentation spectroscopy and infrared spectroscopy were performed on two prototypical guaiacyl (G)-type dilignols containing β-O-4 and β-β linkages, complexed with either lithium or sodium cations. The complexes were generated by nanoelectrospray ionization, introduced into a multistage mass spectrometer, and subsequently cooled in a 22-pole cold ion trap to T ≈ 10 K. A combination of UV photofragment spectroscopy and IR-UV double resonance spectroscopy was used to characterize the preferred mode of binding of the alkali metal cations and the structural changes so induced. Based on a combination of spectral evidence provided by the UV and IR spectra, the Li(+) and Na(+) cations are deduced to preferably bind to both dilignols via their linkages, which constitute unique, oxygen-rich binding pockets for the cations. The UV spectra reflect this binding motif in their extensive Franck-Condon activity involving low-frequency puckering motions of the linkages in response to electronic excitation. In the pinoresinol•Li(+)/Na(+) complexes involving the β-β linkage, the spectra also showed an inherent spectral broadening. The photofragment mass spectra are unique for each dilignol•Li(+)/Na(+) complex, many of which are also complementary to those produced by collision-induced dissociation (CID), indicating the presence of unique excited state processes that direct the fragmentation. These results suggest the potential for site-selective fragmentation and for uncovering fragmentation pathways only accessed by resonant UV excitation of cold lignin ions.

Insight into local structure and molecular dynamics in organic solid-state ionic conductors.

Elucidating the rate and geometry of molecular dynamics is particularly important for unravelling ion-conduction mechanisms in electrochemical materials. The local molecular motions in the plastic crystal 1-ethyl-1-methylpyrrolidinium tetrafluoroborate ([C2 mpyr][BF4 ]) are studied by a combination of quantum chemical calculations and advanced solid-state nuclear magnetic resonance spectroscopy. For the first time, a restricted puckering motion with a small fluctuation angle of 25° in the pyrrolidinium ring has been observed, even in the low-temperature phase (-45 °C). This local molecular motion is deemed to be particularly important for the material to maintain its plasticity, and hence, its ion mobility at low temperatures.

Further insights on the molecular structure, vibrational spectra and ring-puckering potentials of silacyclopent-3-ene and its 1,1-dihalo derivatives: Ab initio and DFT study

The structure, ring-puckering potential energy functions and vibrational spectra of silacyclopent-3-ene (3SCP) and its 1,1-d2 (3SCP-d2), 1,1-difluoro (3SCP-F2), 1,1-dichloro (3SCP-Cl2) and 1,1-dibromo (3SCP-Br2) derivatives were investigated using ab initio MP2 and MP4(SDQ), and BLYP, B3LYP and BB1K density functional calculations. The stable form of 3SCP was obtained from MP4(SDQ) and BB1K levels to be unpuckered with a modest nonplanarity character in excellent agreement with previous experimental results. The 3SCP-F2 ring was predicted by all levels of theory to be the most rigid in the series, while the dichloro and dibromo analogs were predicted to maintain a slightly puckered structure with very small barrier height in terms of the puckering motion. When compared with the results obtained from the MP4(SDQ) theory with the 6-311G(d,p), 6-311+G(d,p) and 6-311++G(d,p) basis sets, the BB1K functional showed noticeable performance over B3LYP and MP2 methods in predicting reliable ring-puckering angle and barrier for all members of the 3SCP series. The infrared and Raman spectra of the investigated cyclic organosilanes were revisited on the basis of the frequencies calculated at B3LYP and BB1K levels and the derived potential energy distribution values.

Molecular structure, conformation, and large amplitude motion of barbituric acid as studied by gas-phase electron diffraction and quantum chemical calculations

B3LYP and MP2(Full) calculations with large basis sets predict the planar equilibrium structure of barbituric acid and reveal large amplitude ring puckering motion which is associated with envelope conformation. The potential function for this motion has only one minimum. This function was used in the analysis of the gas-phase electron diffraction (GED) data in terms of a dynamic model. The least-squares refinements applying dynamic model agree well with the GED data, whereas the agreement is not satisfactory for a planar static model. There is a good agreement between the experimental structural parameters and those from B3LYP/cc-pVTZ calculation. The MP2(Full)/cc-pVTZ values of C–C and C–N bond lengths are 0.02–0.03 A less than the experimental ones. The structure of the molecule is compared with the structures of several related molecules.

Axial and equatorial hydrogen-bond conformers between (CH2)3S and H(D)F: Fourier transform infrared spectroscopy and ab initio calculations

The coexistence of axial and equatorial hydrogen-bonded conformers of 1 : 1 (CH(2))(3)S-HF (and -DF) has been observed in the same adiabatic expansion of a supersonic jet seeded with argon and in a static absorption cell at room temperature. High level calculations computed the axial conformer to be the most stable one with a small energy difference with respect to the equatorial one, in full agreement with previous microwave experiments. On the grounds of band contour simulations of FTIR spectra and ab initio energetic and anharmonic vibrational calculations, two pairs of ν(s) HF donor stretching bands, observed in a series of jet-FTIR spectra at 3457.9 and 3480.5 cm(-1) have been respectively assigned to the axial and equatorial forms of the 1 : 1 complex. In the jet-FTIR spectra series with HF, the assignment of an additional broad band (about 200 cm(-1) higher in frequency with respect to ν(s)) to a 1 : 2 complex has been supported by theoretical investigations. Experimental detection of both axial and equatorial forms of a cyclic trimer has been confirmed by calculated energetic and vibrational properties. The nature of hydrogen bonding has been examined within topological frameworks. The energetic partitioning within the 1 : 1 dimers has been elucidated with SAPT techniques. Interestingly, the interconversion pathway between two 1 : 1 structures has been explored and it was seen that the formation of the 1 : 1 complex affects the interconversion barrier on the ring puckering motion. The band contour analysis of gas phase FTIR experiments provided a consistent set of vibrational frequencies and anharmonic coupling constants, in good agreement with ab initio anharmonic vibrational calculations. Finally, from a series of cell-FTIR spectra recorded at different partial pressures of (CH(2))(3)S and HF monomers, the absorption signal of the 1 : 1 complex could be isolated which enabled to estimate the equilibrium constant K(p) = 0.023 at 298 K for the dimerization.

Room Temperature Solution ENDOR of some Super Stable Fluorocarbon Radicals

Room temperature fluorine electron nuclear double resonance (ENDOR) has been successfully observed for several superstable fluorocarbon radicals ·C(C2F4R)(i-C3F7)2 in solution. Three radicals were employed in which CF3, F, and O-c-C6F10SO3C2F5 were introduced as R, and all the hyperfine couplings (hfcs) obtained by ENDOR were assigned with the help of ESR simulation and ab initio MO calculation. In case of ·C(i-C3F7)3 large 13C and considerable β-fluorine couplings suggest the nonplaner arrangement for the central and three carbons at the β-position, in spite of the fact that all the methyl fluorine show the same hfc. Therefore, a rapid puckering motion along the C3 axis together with the methyl rotation should average the hfc’s of the 18 fluorine nuclei to give the same value. When one of the CF3 groups is substituted with an F nucleus, the five CF3 groups give two hfc values, suggesting some dynamics still exists for the molecular frame. When a large group, O-c-C6F10SO3C2F5, is substituted for CF3, all the five CF3 groups become nonequivalent and the ENDOR signal becomes intensive and sharp even at 290 K, indicating that the molecular frame becomes rigid. The relation between the ENDOR spectra of these systems and the intramolecular dynamics is discussed.

Applicability of the Hindered Rotor Scheme to the Puckering Mode in Four-Membered Rings

The hindered rotor scheme, originally developed for internal rotors in flexible chains (Van Speybroeck, V.; Van Neck, D.; Waroquier, M.; Wauters, S.; Saeys, M.; Marin, G. B. J. Phys. Chem. A 2000, 104, 10939), is extended to puckering motions in four-membered rings. The applicability of the approach is tested in a variety of heterocyclic compounds for which the partition function, entropy, and heat capacity are calculated. The entropy may be substantially altered by a correct description of the puckering mode. The equilibrium puckering angle ranges between 0 degrees and 30 degrees depending on the heterosubstitution X (CH2, O, S, NH, PH, C=O, C=S, C=NH, C=PH) in the four-membered ring.

Laser-induced fluorescence and single vibronic level emission spectroscopy of chiral (R)-1-aminoindan and some of its clusters in a supersonic jet

Two low energy conformers of the chiral (R)-1-aminoindan molecule are identified in supersonic jet and their ground and excited states vibrational spectroscopy has been investigated by laser-induced fluorescence (LIF) excitation and single vibronic level (SVL) emission spectroscopy. Ab initio calculations confirm the existence of two lowest-energy structures, where the amino group is in equatorial position with its lone pair directed opposite to the aromatic electron cloud. Harmonic frequencies have been calculated for these two conformers at the DFT level with B3LYP functional. A low-frequency progression of 118 cm(-1) and 114 cm(-1), respectively, appears in the fluorescence excitation spectrum of the two conformers, with its ground state counterpart at approximately 147 cm(-1). It has been assigned to the puckering motion coupled with the ring flapping mode. The other calculated low-frequency mode corresponds to the puckering motion coupled with the ring twisting mode and its ground state frequency has been observed at 119 cm(-1) and 111 cm(-1) from SVL spectra. Both conformers form similar 1 : 1 water clusters, whose 0-0 transitions are shifted to the blue by 41 cm(-1) and 44 cm(-1), respectively, and whose SVL spectra are similar. Interestingly, one of the conformers seems to preferentially make complexes with (S)-methyllactate, while the other one shows selective complexation to (R)-methyllactate.

Large Amplitude Out-of-Plane Vibrations of 1,3-Benzodioxole in the S0 and S1 States: An Analysis of Fluorescence and Excitation Spectra by ab Initio Calculations

We report the analytical expressions of the two-dimensional potential energy surfaces (PES) spanned by the puckering and flapping vibrations in the S0 and S1 states of 1,3-benzodioxole (BDO). Both PES are obtained from S0 and S1 energies computed on a grid of 2500 molecular geometries at the CASPT2 level. Both the S0 and S1 PES are anharmonic, and the planar geometry corresponds to a barrier that separates two minima at nonplanar geometries along the puckering/flapping deformations. Eigenvalues and eigenvectors of the mixed puckering/flapping modes are calculated by the Meyer flexible model. Improved vibronic levels, in better agreement with the observed spectra, are obtained by suitably optimized CASPT2 surfaces. To assign the lower-energy (0-500 cm(-1)) portion of emission and absorption spectra, we evaluate the band intensities by estimating the Franck-Condon factors between the puckering/flapping eigenvectors of the S0 and S1 states. From these calculations, we obtain a satisfactory assignment of the ground state IR spectra and of the fluorescence excitation spectrum. Both assignments are supported by the analysis of the vibrational structures of several single vibronic level (SVL) fluorescence spectra. The successful interpretation of these spectra shows that the S0 and S1 PES that we derive for BDO are substantially correct. The barrier heights in the two states are similar: 125.7 and 190.4 cm(-1) in S0 and in S1, respectively. In S0, the barrier is associated essentially with the puckering motion. In S1, it involves to a considerable extent also the flapping coordinate, whose vibrational frequency is much lower in S1 than in S0. This fact introduces a substantial Duschinsky effect in the S0-S1 transitions of BDO.

A Two-Dimensional Potential Energy Surface and Associated Quantum States for the Ring-Puckering Vibrations of Two Equivalent Rings: A Study of Bicyclo[3.3.0]oct-1,5-ene

The results of ab initio calculations have been utilized to produce plausible two-dimensional energy surfaces of the form V = a(x14 + x24) + b(x12 + x22) + cx12x22 + dx1x2 for the ring-puckering vibrations of bicyclo[3.3.0]oct-1,5-ene, with each coordinate representing the puckering motion of one ring. Polynomial expressions for the kinetic energy (reciprocal reduced mass) associated with the puckering motions of the rings were developed and utilized. These expressions were used together with the potential energy surfaces to calculate the quantum states and spectroscopic transitions for the puckering motions. The effect of varying the potential energy parameters c (ring-to-ring interaction) and d (cis-trans energy difference) on the energy levels and transitions was examined. The expected spectra are very rich and complex.

A new method for distinguishing between Al 2X 6 (X=Cl, Br) conformers based on ab initio calculated nuclear quadrupole coupling constants

Nuclear quadrupole coupling constants ( χ) of 27Al, 35Cl and 81Br in AlX 3 monomers as well as Al 2X 6 (X=Cl, Br) dimers are calculated at the RHF/6-311G* and B 3LYP/6-311G* levels, using G aussian 98 package. Correlations are made between χ and dihedral angles θ, of Al 2X 6. These θs are produced through the ring puckering motions about the hinge line which joins the two bridge halogens (X b). Nuclear quadrupole coupling constants of 35Cl, 81Br and 27Al are used as probes for monitoring the departure of the symmetry of Al 2X 6 from a high symmetry point group D 2h to a lower one. The χs of chlorine nuclei of AlCl 3 differ significantly from those of Al 2Cl 6. These differences appear negligible for AlBr 3 in comparison to Al 2Br 6. This work demonstrates the considerable sensitivity of nuclear quadrupole resonance in distinguishing between Al 2X 6 conformers. This is in comparison to the usage of energy differences which is customarily employed.

Trimethylene Sulfide···(HCl)n(n= 1, 2) Complexes: A Theoretical Study

The potential energy surfaces (PESs) of the trimethylene sulfide···(HCl)n (n = 1, 2) complexes were explored at the MP2 level of theory by using Pople's and Dunning's basis sets including diffuse functions. Two different axial and equatorial dimer hydrogen-bonded complexes were located on the PESs. The computed geometries are in good agreement with the microwave spectroscopic determination of these two structures. MP4SDTQ//MP2 and QCISD(T)//MP2 single-point calculations were carried out on the significative points on the PESs (minima and transition structures). Comparison among supermolecule uncorrected, counterpoise procedure (CP)-corrected, and BSSE-free complete basis set (CBS) interaction energies suggests that the CP algorithm overcorrects the basis set superposition error (BSSE). However, the supermolecule CP-corrected interaction energies agree rather well with the corresponding BSSE-free symmetry adapted perturbation theory (SAPT) values. The energetic analysis shows that the axial conformer is slightly more stable than the equatorial one, in agreement with the experimental findings. The nature of the interactions in the complexes is analyzed using different theoretical tools: Bader's, NEDA's, Fukui's, and SAPT results are presented and discussed. The interconversion between the two conformers takes place through ring puckering motion involving an energetic barrier of 214−266 cm-1 (QCISD//MP2 level). This small barrier is consistent with the experimental observation that only the axial conformer is detected when argon is used as a carrier gas. The possibility for a trimer structure to be present in the molecular beam was analyzed from the theoretical viewpoint. Structural and energetic information on the trimethylene sulfide···(HCl)2 complex from ab initio calculations can be helpful to plan further experimental work to detect such a trimer structure.

A New Way for IVR Induced by Fast Internal Motion As Revealed by a Jet-Cooled Spectrum of Cyclopentene

The CH stretching spectra of cyclopentene at 25 K in a molecular beam and at room temperature in a gas cell are measured and compared. These spectra are modeled with an Hamiltonian written in curvilinear coordinates and taking into account, in the adiabatic approximation, the coupling of the CH stretching vibrations with the puckering motion and the angle deformations by Fermi resonance. The curves of the vibrational energy calculated as a function of the puckering coordinate exhibit several crossings. From these crossing points, the vibrational energy can follow different ways. The probability of following one or the other way is discussed and determined from the fit of the two spectra. The presence of some stretching vibrations with an unexpected asymmetrical variation is experimentally evidenced. A correct reproduction of the experimental spectra is obtained.