Structure of fenchone by broadband rotational spectroscopy

Abstract

The bicyclic terpenoid fenchone (C10H16O, 1,3,3-trimethylbicyclo[2.2.1]heptan-2-one) has been investigated by chirped pulse Fourier transform microwave spectroscopy in the 2-8 GHz frequency region. The parent species and all heavy atom isotopologues have been observed in their natural abundance. The experimental rotational constants of all isotopic species observed have been determined and used to obtain the substitution (rs) and effective (r0) structures of fenchone. Calculations at the B3LYP, M06-2X, and MP2 levels of theory with different basis sets were carried out to check their performance against experimental results. The structure of fenchone has been compared with those of norbornane (bicyclo[2.2.1]heptane) and the norbornane derivatives camphor (1,7,7-trimethylbicyclo[2.2.1]heptan-2-one) and camphene (3,3-dimethyl-2-methylenebicyclo[2.2.1]heptane), both with substituents at C2. The structure of fenchone is remarkably similar to those of camphor and camphene. Comparison with camphor allows identification of changes in ∠CCC angles due to the different position of the methyl groups. All norbornane derivatives display similar structural changes with respect to norbornane. These changes mainly affect the bond lengths and angles of the six-membered rings, indicating that the substituent at C2 drives structural adjustments to minimise ring strain after its introduction.

Molecular geometries and other properties of H2O⋯AgI and H3N⋯AgI as characterised by rotational spectroscopy and ab initio calculations

“…Observations of rotational transition intensities suggest that the rotational temperature of probed species is of the order of 2 K. The generation of H 2 O· · · AgI was achieved while using a solid target rod composed of silver iodide combined with a bespoke sample holder containing water and placed immediately in front of the nozzle. 24 Attempts to generate H 2 O· · · AgI through vaporisation of a silver rod in the presence of gaseous H 2 O and CF 3 I precursors were unsuccessful. Isotopically enriched samples of 15 NH 3 (Sigma-Aldrich, 98% 15 N), ND 3 (Sigma-Aldrich, 99% D), D 2 O, and H 2 18 O were used to generate and record the spectra of isotopologues as appropriate.…”

Conformational Flexibility of Limonene Oxide Studied By Microwave Spectroscopy

“…The microwave spectrum of limonene oxide was recorded in the frequency range of 4 to 20 GHz by using an impulse Fabry‐Pérot molecular beam Fourier‐transform microwave (FP‐FTMW) spectrometer available in Lille and in the frequency range of 2 to 8 GHz by using a chirp FTMW (CP‐FTMW) spectrometer at King's College London . ( R )‐(+)‐limonene oxide was purchased from Sigma–Aldrich as a mixture of cis and trans isomers (97 % purity) and was used without further purification.…”

“…The microwave spectrum of limonene oxide was recorded in the frequency range of 4t o2 0GHz by using an impulse Fabry-PØrot molecular beam Fourier-transform microwave (FP-FTMW) spectrometer [27][28][29][30] available in Lille and in the frequency range of 2t o 8GHz by using ac hirp FTMW (CP-FTMW) spectrometer at King's College London. [24] (R)-(+ +)-limonene oxide was purchased from Sigma-Aldrich as am ixture of cis and trans isomers (97 %p urity) and was used without further purification. In both experiments, af ew droplets of neat liquid were placed in ab espoke heating nozzle and heated at temperatures of 343 to 353 Kt oo btain optimal signals, as limonene oxide has al ow vapor pressure at room temperature.…”

“…[10,13] To this end, techniques such as two-dimensionalh eteronuclear NMR spectroscopy, [14] Fourier-transform infrared spectroscopy (FTIR), [15] proton-transfer reactionm ass spectrometry (PTR-MS), [16] high-performance liquid chromatography (HPLC), [15] and gas chromatography coupledw ith mass spectrometric detection [17,18] have been widely applied. Recently,h igh-resolution microwaves pectroscopyw as also employed to study the structure in gas phase of several terpenoids, namely,p erillaldehyde, [1] carvone, [19,20] limonene, [19] carvacrole and thymol, [21] menthone and menthol, [2] 4-carvomenthenol, [22] camphor, [23] and fenchone, [24] among others.…”

The role of secondary interactions on the preferred conformers of the fenchone–ethanol complex

“…In particular, rotational spectroscopy is one of the most powerful methods to obtain precise structural information on small biomolecular systems, and it has the advantage of identifying different coexisting conformations, thus showing the system's flexibility (see, for example refs. [7][8][9][10][11] ). The development of chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy 12 , which allows fast collection of broad segments of the spectrum at once, facilitating the identification of spectral patterns, has dramatically extended the range and complexity of molecular systems that can be tackled, including large molecules 13 , odorants 14,15 , and intricate clusters and complexes [16][17][18] .…”