[Ingeniosuccinimide]

I'm reading something on ²J (geminal) coupling in various compounds and there is either something quite confusing or something I understood very badly, because I don't understand the following:

why do compounds like methane and 1,1-dichloroethene have a negative value of ²J coupling (-12 and around -4 Hz, respectively) and what does it mean in general that the coupling constant has a negative value?

What is more confusing, how is it even possible that compounds that have chemically equivalent hydrogens which are not enantiotopic (like upper two) can have any geminal H-H coupling constant at all?? I mean, when we look at the ¹H spectra of methane and 1,1-dichloroethene, both spectra contain only one singlet, no first- or higher-order spectra. Why do the coupling constants here exist?

[Babcock_Hall]

To be honest, I am not sure of the answer.  However, I sometimes find it valuable to differentiate between coupling and splitting.  The geminal hydrogen atoms of methane and 1,1-dichloroethene are (to the best of my knowledge) coupled, but because they have the same chemical shift, no splitting is observed.  This is true even of hydrogens that are adventitiously equivalent in chemical shift.

[Irlanur]

1) Depending on the bonding situation, parallel or anti-parallel alignment of the spins is energetically favoured, that's why J can be positive or negative.
2) I actually don't know how the J-couplings of magnetically equivalent protons as in DCM are practically obtained, but think about this. The two protons are only equivalent in an isotropic environment or if the molecule tumbles very fast. in the solid state, the orientation of the nuclear spins to the B-field can be different for the 2 protons in DCM, so in principle, a coupling can be measured.

[Dan]

2) I actually don't know how the J-couplings of magnetically equivalent protons as in DCM are practically obtained, but think about this. The two protons are only equivalent in an isotropic environment or if the molecule tumbles very fast. in the solid state, the orientation of the nuclear spins to the B-field can be different for the 2 protons in DCM, so in principle, a coupling can be measured.

The original question was regarding 1,1-dichloroethene, not DCM.

[Babcock_Hall]

"The molecular orbital perturbation theory of Pople and Bothner-By (J. Chem. Phys. 1965, 42, 1339) predicts the electronic effects of substituents on these coupling constants based on the interaction between filled and empty orbitals of the CH2 fragment."
http://www.chem.wisc.edu/areas/reich/chem605/
Look under section 5.0.4.