title: Methyl tunnelling sidebands in the low-field NMR spectrum of 3-pentanone: Driving A-E transitions using rf irradiation.
abstract: Using magnetic field-cycling at cryogenic temperatures, low-field dipole-dipole driven NMR spectra have been recorded on 3-pentanone (CH3CH2C(O)CH2CH3). The spectra are characterised by tunnelling sidebands arising from the quantum dynamics of the methyl (CH3) rotors. From the sideband frequencies, the CH3 tunnelling frequency is determined to be νt=3.05±0.01MHz. The tunnelling sidebands are characterised by A-E transitions in nuclear spin-symmetry, involving simultaneous changes in tunnelling and nuclear spin states. To gain further insight, a theoretical analysis of the spin Hamiltonian matrix has been used to calculate the sideband transition probabilities. These are subsequently used in a thermodynamic model to simulate the low-field NMR spectrum which is compared with experiment. The level-crossings encountered as part of the magnetic field-cycling NMR sequence are found to play an essential role in determining the tunnelling sideband intensities.
authors.affiliation: School of Physics & Astronomy, University of Nottingham, Nottingham NG7 2RD, UK.
authors.forename: Bo
authors.initials: B
authors.lastname: Zhang
authors.affiliation: School of Physics & Astronomy, University of Nottingham, Nottingham NG7 2RD, UK. Electronic address: a.horsewill@nottingham.ac.uk.
authors.forename: Anthony J
authors.initials: AJ
authors.lastname: Horsewill
keywords: CH(3) dynamics; Field-cycling NMR; Methyl tunnelling; Quantum rotor