  We construct nontrivial cohomology classes of the space $Imb(S^1,\R^n)$ of
imbeddings of the circle into $\R^n$, by means of Feynman diagrams. More
precisely, starting from a suitable linear combination of nontrivalent
diagrams, we construct, for every even number $n\geq 4$, a de Rham cohomology
class on $Imb(S^1,\R^n)$. We prove nontriviality of these classes by evaluation
on the dual cycles.
