LETTER doi:10.1038/nature12150 Enantiomer-specific detection of chiral molecules via microwave spectroscopy David Patterson1, Melanie Schnell2,3 & John M. Doyle1 Chirality plays a fundamental part in the activity of biological mole- signal can be accurately predicted via numerical integration of the well- cules and broad classes of chemical reactions, but detecting and known asymmetric-top Hamiltonian (Fig. 2b). quantifying it remains challenging1. The spectroscopic methods We validate our technique using R-, S- and racemic 1,2-propanediol. of choice are usually circular dichroism and vibrational circular This molecule was chosen because the relevant molecular constants dichroism, methods that are forbidden in the electric dipole are well characterized (A 5 8,572.05 MHz, B 5 3,640.10 MHz, C 5 2 approximation . The resultant weak effects produce weak signals, 2,790.96 MHz, ma 5 1.2 D, mb 5 1.9 D, mc 5 0.36 D; ref. 14) and because and thus require high sample densities. In contrast, nonlinear tech- it is readily available commercially in enantiopure form. 1,2-propanediol niques probing electric-dipole-allowed effects have been used for and larger molecules occupy a large number of quantum states at room sensitive chiral analyses of liquid samples3–7. Here we extend this temperature, which dilutes the signal obtained from a single rotational class of approaches by carrying out nonlinear resonant phase- level (for example, more than 5,000 occupied states for 1,2-propanediol). sensitive microwave spectroscopy of gas phase samples in the pres- To reduce the number of occupied states and increase the resonant ence of an adiabatically switched non-resonant orthogonal electric polarizability of the sample, we therefore substantially cool the molecu- field; we use this technique to map the enantiomer-dependent sign lar gas using techniques developed previously and discussed in detail of an electric dipole Rabi frequency onto the phase of emitted elsewhere15: the warm molecules are injected into a cryogenic buffer microwave radiation. We outline theoretically how this results in gas cell thermally anchored to a closed-cycle pulse-tube refrigerator a sensitive and species-selective method for determining the chir- and cooled to a temperature of ,7 K. However, other sources of cold ality of cold gas-phase molecules, and implement it experimentally molecules could also be used. A natural choice would be a pulsed to distinguish between the S and R enantiomers of 1,2-propanediol supersonic beam source (rotational temperature ,2 K), which is fre- and their racemic mixture. This technique produces a large and quently used as a cold molecule beam source for Fourier transform definitive signature of chirality, and has the potential to determine microwave spectroscopy16. the chirality of multiple species in a mixture. The experimental set-up, sketched in Fig. 3, has two walls of the Our approach to determining chirality depends only on the parity cryogenic cell formed by mirrors that comprise a tunable plano-concave conserving Hamiltonian of an asymmetric top in an external electric Fabry–Pe´rot microwave cavity. The cavity is used to excite and detect the 8 field : the three rotational constants A, B and C and the corresponding molecules, with each of its transverse and longitudinal spatial modes dipole moment component magnitudes jmaj, jmbj and jmcj determine supporting two (degenerate) modes of orthogonal polarization. These the rotational energy levels of such a molecule, and further specifica- modes can be separately addressed via waveguides attached to the planar tion of the signs of ma, mb and mc fully determines its chirality (Fig. 1). mirror and coupled to the cavity via apertures (denoted A and B in The sign of any two of the three dipole moment components ma, mb and m is arbitrary and changes with the choice of axes, whereas the sign of c R-1,2-propanediol S-1,2-propanediol the combined quantity mambmc is axis independent and changes sign with enantiomer. Equivalently, any measurement of the combined quantity mambmc, and indeed any measurement of chirality, is time- even and parity-odd. In the context of direct molecular chirality detec- tion, parity violating energy shifts arising from the weak interaction have been predicted9–12 and could in principle distinguish enantio- mers; but the resultant very small frequency shifts have never been observed and are not relevant to the techniques demonstrated here. A A The Rabi frequency describing an electric dipole transition between μa μa rotational states of a chiral molecule differs in sign for opposite μb 13 μb enantiomers , and we demonstrate here theoretically and experiment- C C μc ally that it can be mapped onto the phase of emitted radiation by μc applying an electric field that varies during the molecular coherence B B time. This switched electric field, Ex, combined with an applied micro- wave field, Ez, induces ^y-polarized oscillations of the molecules’ dipole, causing them to emit radiation polarized along the y axis, orthogonal to both E and E (Fig. 2). This radiation is detected and amplified, with Figure 1 | The two enantiomers of chiral 1,2-propanediol. The Hamiltonian x z of a rigid chiral molecule in an external electric field is enantiomer-dependent. w the extracted signal phase differing by 180u between left- and right- In any chiral molecule, opposite enantiomers have the same rotational handed (S and R) enantiomers and indicating the dominant enantiomer constants A, B and C, and the same magnitude of dipole moment components (that is, the sign of the enantiomeric excess), while the signal magnitude | ma | , | mb | and | mc | , but the sign of the combined quantity mambmc is distinct for V indicates the magnitude of the enantiomeric excess. With known each enantiomer, independent of choice of axes. The displayed orientation of molecular constants A, B, C, ma, mb and mc, the enantiomer-dependent the molecules is for illustrative purposes. 1Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA. 2Center for Free-Electron Laser Science, D-22607 Hamburg, Germany. 3Max-Planck-Institut fu¨ r Kernphysik, D-69117 Heidelberg, Germany. 23 MAY 2013 | VOL 497 | NATURE | 475 ©2013 Macmillan Publishers Limited. All rights reserved RESEARCH LETTER a 1101 E = 12,212 MHz y 1100 Cold cell (7 K) 1 z (into page) E t 10–1 z cos(ω ) a-type 1111 Helium fill line x 12,212 MHz E = 11,363 MHz 1110 111–1 Mirror 2 Mirror 1 Waveguide A Switchable microwave e z polarization power amplifier -type c b-typ To phase sensitive detection Waveguide B Molecule Low noise Switched field Ex y polarization E source (300 K) amplifier 0000 = 0 b 2 E P Applied fields Induced polarization, y Electrical isolation E S 1.5 z (mixed down) enantiomer (0.1 mm G10) ±500 V E /100 R enantiomer ) 1 x Figure 3 | A cryogenic buffer gas Fourier transform microwave –1 spectrometer that provides enantiomer-specific detection. Molecules are introduced into a cryogenic cell from a room-temperature tube held close to an 0.5 aperture in mirror 1. Microwave radiation is introduced using waveguides via field (V cm coupling apertures in mirror 2. This mirror can be rapidly switched between (arbitrary units) E 0 y P 6500 V and ground, applying a time-varying electric field in the ^x direction; it is insulated from the cell by G10 fibreglass. The molecules are polarized initially in the –0.5 R- ^z direction via a linearly polarized microwave pulse coupled from waveguide A. The S- resulting enantiomer-specific radiation is coupled out of the cavity via waveguide B, –1 which is oriented to detect ^y-polarized microwaves. 400 600 800 1,000 1,200 1,400 Time (ns) Emw is the field strength of the applied microwave field in the z direction. Figure 2 | 1,2-propanediol as a rigid rotor. a, The relevant rotational level The excitation frequency v is tuned to theji 000 ?ji110 rotational trans- structure of 1,2-propanediol. Each state is designated with jiJk{1k1m . Molecules ition of the ground-state conformer of 1,2-propandiol at 12,212 MHz. initially in the ground state are prepared in a superposition ofji 0000 andji 1100 (We use the jiJk{1k1 nomenclature of ref. 8, where k21 and k1 indicate the via a c-type microwave transition. A change in the electric field Ex mixes in quantum numbers of the limiting prolate and oblate symmetric top { m components ofji 1101 andji 110 1 with complex phases proportional to a. respectively; when m is specified, we use jiJ ). Allowed electric dipole radiation between these admixed states and the ground k{1k1m state produces an oscillating electric field in the laboratory ^y direction via a The maximum magnitude of Ez and the pulse length t are adjusted V t = p V b-type transition. In the relevant limit of small Ez and Ex, this electric field Ey is to make j j /2 for all molecules, where is the Rabi frequency. proportional to mambmc and therefore changes sign with enantiomer. The This ^z-polarized microwave pulse induces an oscillating electric dipole 21 applied field Ex of 65 V cm induces a 500 kHz Stark splitting in theji 110m polarization, Pz, in the molecular ensemble. The electric field Ex is set to states that is not relevant to the dynamics due to the short (,200 ns) duration of zero 200 ns after the end of the microwave pulse; Ex is switched over the field. Admixing with theji 101 state at 6,431 MHz (not shown) also about 100 ns—rapidly compared to the molecular dephasing but contributes a small amount to the chiral signature. b, A simulation of 1,2- slowly compared to v.