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Hamiltonians for Floppy Molecules (as needed for FIR astronomy) A broad overview of state-of-the-art successes and failures for molecules with large amplitude motions (LAMs) Jon T. Hougen Sensor Science Division, NIST Gaithersburg, MD 20899-8441.
(as needed for FIR astronomy)
A broad overview of state-of-the-art successes and failures for molecules with large amplitude motions (LAMs)
Jon T. Hougen
Sensor Science Division, NIST
Gaithersburg, MD 20899-8441
What can experiment and Hamiltonians for floppy molecules “easily” provide, that FIR astronomers might want?
Effective Hamiltonians (Heff) are the only tools available at the moment for fitting and calculating (interpolating) line positions to experimental accuracy.
In general ab initio methods cannot provide kHz or MHz accuracy. Effective Hamiltonians sacrifice physics “purity” by using “empirical” quantum mechanical interaction terms to get experimental accuracy in their Heff calculations.
But effective Hamiltonians cannot be used at present for all molecules with large-amplitude motions (LAMs).
We therefore now survey examples of:
(i) classes of vibrational states and
classes of molecules that can be accurately treated, and examples of:
(ii) classes of states and molecules that cannot be accurately treated by Heff’s.
Rotational levels (& therefore transitions) in ground vibrational states can often be treated very well.
Rotational levels (& therefore transitions) in excited vibrational states can only be treated “well” if the state is “isolated” from other vibrational states.
Periodic motions = internal rotations
Reciprocating motions (back and forth) =
= inversions, proton exchange, etc.
Periodic motions are easy to treat, using
rapidly converging (short) Fourier series
for V and .
Reciprocating motions are hard to treat, using slowly converging (long) power series for V and .
1. Only periodic motions (Fourier series in H = T + V and in wavefunctions).
2. Only reciprocating motions (power series in H = T + V and in wavefunctions).
3. Both periodic & reciprocating motions
(high-barrier tunneling H = T + V).
Make some general remarks in the next few slides on molecules in each class.
Molecules with only one symmetric-top internal rotor LAM are fit to 1 to 20 kHz. Calc’d intensities are probably OK to 20%.
Three-fold barrier (e.g., methanol)
Six-fold barrier (e.g., toluene)
Any point group symmetry at equilibrium
My favorite computational method: Use
Herbst’s procedure for any barrier height, but limited by computation time to low J values = low temperatures T < 100 K.
Molecules with only one asymmetric-top internal rotor (e.g., CH2DOH or CD2HOH):
Hilali, Coudert, Konov, and Klee report in JCP 135 (2011) a fit of 76 torsional subband centers in CH2DOH with rms = 0.09 cm-1>> 20 kHz.
This is good enough for low-resolution FIR astronomical observations, but probably not good enough for anticipated higher-resolution data.
Molecules with only two symmetric-top internal rotors as LAMs are partly under control (to kHz), but no intensities. Programs now available (year or no) are:
Point group symmetries Cs (2010) and C1(no) for 2 inequivalent rotors, with PI group = G18, e.g., methyl acetate.
Point group symmetries C2v (2012) and C2h(no) for 2 equivalent rotors, with PI group = G36, e.g., acetone, dimethyl ether.
Molecules with only one reciprocating LAM and a low barrier (like NH3) are much more difficult. For example,
1980’s papers by Š. Urban and coworkers report calculations of ground state energy levels to 0.001 cm-1 = 30 MHz. Rumor has it that Urban has plans for 2013-2015 to improve these calculations to 10 KHz using both new sub-mm measurements and a better Heff.
Molecules with only one reciprocating LAM and one, two or three symmetric-top internal rotors (e.g., methyl amine with 1 CH3 top & 1 NH2 inversion; or e.g., methyl malonaldehydewith 1 CH3 top & 1 H atom transfer C-O-H O=C).
Only an empirical tunneling formalism is available now for calculating lines to kHz accuracy, but the formalism is applicable only if the barriers for all LAMs are high.
C2H3+ ground state rotational levels are doable to kHz accuracy, because the barrier to H migration is high.
CH5+ ground state rotational levels are NOT doable to kHz accuracy, because the barriers to H roaming are low.
C-H fundamental rotational levels are not doable because of dark state bath.
What is needed in the next 20 years to produce floppy molecule line lists with 30 MHz = 0.001 cm-1 accuracy for FIRastronomy, or alternatively with 30 kHz accuracy for THz astronomy?
High-resolution molecular spectroscopists will need:
1. Better experimental and Int. data,
2. More powerful and accurate Heff’s,
3. Faster computers.