this is a terrible thing

[azz]

a. This is a terrible thing not to be able to see him.
b. This is a bad blow to me not to be able to see him.

Are these sentences correctly punctuated?
Would you say that we need a comma before 'not'?

The idea in (a) is that the terrible thing = not to be able to see him.
The idea in (b) is that the bad blow = not to be able to see him.

Many thanks

[Erik_Kowal]

Yes to the comma if you wish to indicate an emphatic pause.