[Petra988]

what is the difference between INEPT and HSQC? Both experiments provide correlation between the carbon and its attached proton... And about NOESY vs ROESY - all i know about the difference that the first one is good for small and big molecules, crosspeaks are in phase, the second for the medium compounds...

[Babcock_Hall]

INEPT is usually run as a 1D experiment.  HSQC is usually run as a 2D experiment, except when you are setting it up.  Then it is run as 1D.  My recollection on NOESY versus ROESY is not entirely clear, but I seem to recall that there is a difference regarding the sign of the cross peaks under certain circumstances.  You may know that cross peaks can occur either from nuclei that are close in space or from nuclei that are undergoing chemical exchange.  That is the point where the sign difference may manifest itself.

[phth]

For medium 2-4k Da molecules NOE depends on shape, how rigid the molecule is weighted average of the shape of the combination of all conformers.  Also viscosity (dipole) of the solvent.   The NOESY crosspeak sign will tell you if the NOE effect is adding or subtracting relative to the diagonal crosspeak: -I(a)_zcos(Ωt₁) -I(a)_zcos(Ω(a)t₁) [diagonal] + R_abτ_mI(b)_zcos(Ω(a)t₁) [crosspeak].  If H_a experiences negative NOE then it adds to H_b's signal.  R_ab is the cross-relaxation rate.  For small molecules R_ab is linear i.e. w(larmor frequency)*τ(tumbling rate)<<1 where double quantum relaxation dominates(ββ<->αα).  For very large molecules ωτ>>1 where zero quantum relaxation dominates (αβ<->βα), and R_ab gains a negative coefficient making the crosspeaks and the diagonal peaks the same sign.  For the medium sized molecules ωτ≈1  DQR and ZQR are about the same which depends on field strength and the stuff mentioned above, and the signal goes down into the noise.  The ROESY pulse sequence puts the spins on the X' axis and the signal becomes double quantum only.

[Irlanur]

An HSQC is basically a double-INEPT. You put the Polarisation from H to the X spins, let it evolve (indirect dimension) then put it back to H and detect the fid (direct dimension).