The Washington Hospital Center yesterday unilaterally declared an impasse in negotiations with its striking registered nurses and announced it was putting into effect the terms of a proposed contract that the strikers rejected by a 2-to-1 margin Wednesday night.

Hospital spokeswoman Jane Snyder said, "We feel we are at an impasse now that there are 167 people supporting a continuing strike. And as is our legal right, we will place into effect all benefits contained in the contract that was rejected."

As of yesterday morning, 60 percent - of 255 - of the center's 425 registered nurses were honoring the picket lines that went up 21 days ago. The striking nurses voted 167 to 80 to reject the contract offer and then adopted unanimously a motion changing the vote to rejection by acclamation.

Snyder said the contract, which does not include the key union shop provision demanded by the District of columbia Nurses' Association, is being implemented "because we do not penalize the people who are working" by preventing them from receiving the offered benefits.

"Obviously it has no effect on the people who are striking, but we have a lot of nurses in the hospital who are working hard to take care of patients and it seems unfair to penalize them," Snyder said.

Hospital Center Nurses' Association president Dottie Hararas said early yesterday that the contract was rejected largely because of the absence of a union-shop provision.

Members also were said to be displeased with the contract's establishment of a committee to study, and implement, a schedule of some permanent day shifts by Jan. 1, 1979, rather than implementing such a system immediately. There also was discord over some of the contract's seniority provisions.

Snyder said yesterday that the hospital center does not want a union shop because it thinks compulsory union membership for nurses would directly affect the delivery of patient care.

A union shop clause, she said, would "take the control over people who render health care out of the hands of the hospital, which is legally and morally responsible for patient care" and is responsible to the board of trustees. It would put such control in the hands of the union, she asserted.

Strikers have contended that a union shop is the only thing that will guarantee them some autonomy and protect their rights as workers under the contract.

Federal mediator Harold Mills, who has been conducting negotiations, said he expects to call the nurses and the hospital center back to the bargaining table by early next week.