N,N-Dimethylformamide: An interesting case
Here's N,N-Dimethylformamide in the gas phase compared to several ab initio
computations using the structure found by Kimball.exe with standard parameters:
Dimethylformamide - Energies
---------------------------------------------------------------------------------------
Kimball Gamess/6-31g Gamess/6-31g,opt G94W/6-31g,opt
Nucleus-Nucleus Vnn : 180.23058 180.23066 180.08970 180.08979
Electron-Electron Vee : 256.72510 264.77374 264.76824 264.76824
Nucleus-Electron Vne : -929.98466 -938.69044 -938.70460 -938.70460
Electronic energy Eel : -426.74952 -426.92614
potential energy V : -493.02898 -493.68604 -493.84666 -493.84667
kinetic energy T : 246.51004 246.87115 247.01022 247.01022
total energy Etot : -246.51894 -246.81489 -246.83644 -246.83644
Virial ratio -V/T : 2.000036 1.99977 1.99930 1.9993
Coordinates:
NC(CH3) Å : 1.4743 ¦ 1.4612 (avg.)
NC(CHO) : 1.4743 ¦ 1.4137
CO : 1.2102 ¦ 1.2054
CH(CH3) : 1.1003 ¦ 1.0843 (avg.)
CH(CHO) : 1.1003 ¦ 1.0900
OCN deg : 125.73 ¦ 124.30
HCH(CH3) : 112.39 ¦ 108.85
OCH(CHO) : 111.91 ¦ 119.60
CNC(CH3,CH3) : 106.36 (C's have ¦ 115.61
C(CH3)NC(CHO) : 106.36 equal param.)¦ 114.60
Dimethylformamide - Energies (Hartree) This Trace file from Kimball.exe
------------------------------------------------- comes from a second run of DMA
which created the figure at the top.
One electron energy : -684.81120 The numbers are slightly different
Two electron energy : 257.85617 from the comparison table above.
Nuclear repulsion Energy : 180.41847 In this run C4's distance from N1
----------- has been variied independently from
Total Energy Etot : -246.53656 the N1C2 distance. Furthermore the
p-clouds were allowed to move along
Nucleus-Electron pot.Energy Vne : -931.33467 the CO-bond. This created 22 variables
Electron-Electron pot.Energy Vee : 257.85617 for the optimization and a slightly
Nucleus-Nucleus pot.Energy Vnn : 180.41847 more stable result. Several distances
----------- and angles have changed a bit. However
Total potential energy V : -493.06003 the N1C4 bond has not moved near the
Total kinetic energy T : 246.52348 much lower value of the ab initio
Virial ratio -V/T : 2.00005 optimized structures. The reason is,
that Kimball presently cannot model well the
Electronic energy Eel : -426.95503 partial NC4 double bond character of the
Electron potential Vel : -673.47850 NC=O moiety. It gives, however, correctly
the planar arrangement of NCHO. The figure
DMA2 - Interatomic Distances and Angles at bottom contains the numbered classes,
referred to, and shows a better view on
NC(CH3) : 1.4716526 Å the NCHO region. The p-bond has moved
NC(CHO) : 1.4699199 Å nearer the C4 core and not, as perhaps
CH(CH3) : 1.0987654 Å expected, towards the more electronegative
CH(CHO) : 1.0727132 Å O core It appears that the nearby N-atom
CO : 1.2184562 Å and repulsion by the two compact oxygen
OCN : 122.57056 deg lone pairs make up for this result.
CNC(CH3) : 106.71502 deg At the next step an sp2 N-Atom with two
C(CHO)NC(CH3) : 106.71502 deg pz elctrons would share in a three-center
OCH : 113.50472 deg 4-electron p-bond system with C and O. This
NCH(CH3) : 106.70849 deg would generate a planar(C2)NCO moiety as
HCH : 112.08696 deg obtained with most ab initio calculations, see
the discussion on p. 105/106 in "Exploring
DMA2 - Cartesian Coordinates Chemistry with Electronic Structure Methods",
2nd ed., J.B. Foresman & Ae. Frisch, Gaussian
12 Inc. Pittsburgh, 1996.- Only the most exact
model chemistries with RHF-SCF give a nonplanar
N 0. 0. 0. geometry, as known to be true from experiment
C 0.42326436 -1.409471 0. Interestingly Sto-3G monitors a transition state
C -1.4716526 0. 0. for the planar arrangement. Kimball is, of
H 1.5220297 -1.4093458 0. course, too much nonplanar as expected from
H 1.0165492E-2 -1.8633589 -0.91138382 a pyramidal N-node with the lone pair at one
H 1.0165492E-2 -1.8633589 0.91138382 corner, see figure below.
H -1.7875504 1.0523754 0.
H -1.7875504 -0.52618769 0.91138382
H -1.7875504 -0.52618769 -0.91138382
C 0.42276603 0.56837388 -1.2879769
O 1.1483951 1.543931 -1.3678916
H 0.12947952 0.17406669 -2.2415064
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