Rotational coherence of encapsulated ortho and para water in fullerene-C60 revealed by time-domain terahertz spectroscopy

Abstract

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

Exploring the Properties of H2O@C60 with the Local Second‐Order Møller‐Plesset Perturbation Theory: Blue or Red Shift in C60 and H2O Fundamentals to Expect?

“…Assuming a linear arrangement of this bond along the line which connects two opposite carbon atoms of a C 60 cage and taking into account the van der Waals radii of carbon, oxygen and hydrogen atoms of 1.75, 1.40 and 1.17 Å, [67] respectively, one gets the optimal distance of 7.03 Å (1.75+1.40+0.96+1.17+1.75) needed to fit the H 2 O molecule inside the cage without causing any strain. This value is only slightly lower than the maximum separation between the two opposite carbon atoms in C 60 (7.09 Å with DF‐LMP2/cc‐pVTZ) indicating that the water molecule indeed has sufficient room to adopt different orientations inside the cage and to rotate freely (what has in fact been observed experimentally [26,28,31] ). On a theoretical side, such a flexibility of the water molecule should lead to the appearance of almost isoenergetic H 2 O@C 60 isomers which are not separated by tangible energy barriers.…”

“…In addition, the electronic conductivity of a single-molecule junction based on H 2 O@C 60 is decreased when compared to that in empty C 60 due to the change in orbital energy levels. [25] H 2 O@C 60 has been the subject of numerous spectroscopic investigations utilizing nuclear magnetic resonance, inelastic neutron scattering, infrared and terahertz spectroscopies [21,[26][27][28][29][30][31][32] to elucidate the role of quantum effects in the dynamics of the encaged water molecule. Similar to H 2 @C 60 , the H 2 O@C 60 complex also demonstrates unusual nuclear spin relaxation, [30] ortho-para spin conversion [26][27][28]31,33] (which influences the lowtemperature heat capacity of the complex [34] ), and strong quantum effects due to quantization of the guest's translational, rotational and vibrational degrees of freedom with the next coupling of translational-rotational motions (for a more comprehensive discussion please refer to the recent articles [15,35,36] ).…”

“…[25] H 2 O@C 60 has been the subject of numerous spectroscopic investigations utilizing nuclear magnetic resonance, inelastic neutron scattering, infrared and terahertz spectroscopies [21,[26][27][28][29][30][31][32] to elucidate the role of quantum effects in the dynamics of the encaged water molecule. Similar to H 2 @C 60 , the H 2 O@C 60 complex also demonstrates unusual nuclear spin relaxation, [30] ortho-para spin conversion [26][27][28]31,33] (which influences the lowtemperature heat capacity of the complex [34] ), and strong quantum effects due to quantization of the guest's translational, rotational and vibrational degrees of freedom with the next coupling of translational-rotational motions (for a more comprehensive discussion please refer to the recent articles [15,35,36] ). However, the global-minimum structure of H 2 O@C 60 has not been unequivocally determined by various theoretical calculations up to now leading to an open question of the appearance of blue/red shifts in vibrational frequencies of the water molecules caused by encapsulation.…”

Encapsulation of a Water Molecule inside C60 Fullerene: The Impact of Confinement on Quantum Features

“… 1 − 10 In view of exciting properties and the prospects of applications, as new energy storage materials, among others in nanoscience, these species were revealed to exhibit some unique and unexpected properties, which triggered their extensive investigation from both theoretical and experimental sides. 2 , 11 − 31 …”

Terahertz Optical Properties of KTiOPO4 Crystal in the Temperature Range of (−192)–150 °C

“…It is important for the development of small-size terahertz (THz) radiation sources tunable in a wide frequency range and possessing high spectral brightness. Among the tasks that could be solved with the help of such sources one should notice gas analysis, in particular, the construction of a terahertz Light Detection and Ranging (LiDAR) for monitoring minor gas components in the ground layer of the atmosphere on kilometer-length paths for the environment and climate control [9,10]; investigation of nuclear spin isomers of molecules and their conversion [11,12]; development of compact accelerators for charged particles [13]; the advancement of nonlinear optics to new spectral ranges [14] and study of selective effects of THz radiation on living organisms [15].…”